Learn Hardcore Japanese: aikon ni ha mori no syasin wo tukatte ite, sizen ga suki da to hitome de wakaru you ni natte imasu. - For the icon I use a photo of a forest, so you can see at a glance that I like nature.

に here marks the target / destination / role:
アイコンに〜を使う = “to use ~ for the icon / as the icon.”
は makes that part (アイコンに) the topic or adds a slight contrast:
“As for (what I use) for the icon, ...”

So アイコンには森の写真を使っていて literally feels like:
“As for the icon, I’m using a photo of a forest, and…”
or
“For the icon, I use a forest photo, and… (as for that part of my profile)”

The には often appears when you want to say “as for X (in the role of Y, at Y, to Y, etc.)…”

Why is アイコン marked with に and not を or just は?

The verb is 使う (“to use”), and in Japanese there is a common pattern:

A に B を使う = “use B for A / use B as A.”

Examples:

パンにジャムを使う = use jam for bread.
タイトルに赤い文字を使う = use red letters for the title.

So:

アイコンに森の写真を使う
= “use a forest photo for the icon / as the icon.”

If you said アイコンを森の写真に使う, it would sound like “use the icon as a forest photo,” which is backwards.

You could make アイコン just a topic with アイコンは, but then you’d lose the idea of “for/as the icon.” The に is needed for that “role/target” meaning; は then attaches to that phrase to topicalize it: アイコンには.

What does 森の写真 mean exactly? Why is の used here?

森の写真 literally is:

森 = forest
森の = “forest’s” / “of a forest” (descriptive)
写真 = photograph

So 森の写真 = “a photo of a forest”.

The particle の connects two nouns, where the first one describes or belongs to the second:

海の写真 = a photo of the sea
家族の写真 = a photo of my family
冬の写真 = a winter photo (photo of winter scenery)

You can’t say 森写真 in Japanese; you need の to link them.

Why is it 使っていて and not just 使って or 使っています?

使っていて is:

使って (te-form of 使う)
＋いる (progressive / ongoing state)
In te-form again to connect to the next part: 使っていて、〜

Nuance:

使っている: “is using / is in a state of using.”
使っていて、〜: “(I) am using it, and as a result / while doing that, ~”

So アイコンには森の写真を使っていて、自然が好きだと一目で分かるようになっています:

“For the icon, I’m using a forest photo, and because of that, it is set up so that you can tell at a glance that I like nature.”

If you used:

使って、自然が好きだと一目で分かるようになっています。
This is okay, but 使っていて sounds more like a current ongoing design/state, less like a simple one-time sequence.
使っています at the start of the sentence would usually end a sentence:
アイコンには森の写真を使っています。自然が好きだと一目で分かるようになっています。
(Two separate sentences instead of one complex one.)

Who is the subject of this sentence? Where is “I” or “you”?

The subject (like “I,” “you,” or “they”) is omitted, which is very common in Japanese.

From context, it’s most natural to understand:

“For my icon, I’m using a forest photo, so that you can see I like nature at a glance.”

But grammatically, the sentence itself is just describing the setup:

“For the icon, a forest photo is used, and it is structured so that one can tell at a glance that nature is liked.”

Japanese relies on context, so the actual pronoun (私, 僕, あなた, etc.) often doesn’t appear unless needed. The sentence focuses on the icon and its design, not on explicitly saying “I.”

Why is it 自然が好き and not 自然は好き?

With the adjective-like word 好き, the thing you like is normally marked by が:

私は音楽が好きです。= I like music.
彼は犬が好きです。= He likes dogs.

So:

自然が好きだ = “(I) like nature.”

Using 自然は好き is not impossible, but は would add a contrast/topic nuance:

自然は好きだけど、虫は嫌いです。
= I do like nature, but I hate insects.

In this sentence, it’s just stating the fact “likes nature,” without contrast, so 自然が好きだ is the normal, neutral pattern.

Why is it 好きだと分かる and not 好きですと分かる?

When you have “Xだと分かる” (understand/see that X is the case), the だ before と is usually in plain form, even in otherwise polite sentences:

忙しいと分かります。 (plain adjective)
静かだと分かります。 (plain な-adjective + だ)
学生だと分かります。 (plain noun + だ)

Similarly:

自然が好きだと分かる。

Using 好きですと分かる is grammatically possible but sounds a bit stiff or unnatural in most everyday contexts. In embedded clauses like this, plain form before と is the standard pattern. The overall politeness comes from the last verb 〜分かるようになっています (polite います).

What exactly does 一目で mean, and what is the role of で here?

一目で is:

一目 (ひとめ) = “one look / one glance”
で = particle indicating means, method, or state (“by / with / in the state of”)

So 一目で literally means:

“with one glance / by one look” → naturally translated as “at a glance.”

Similar patterns:

一回で分かる = understand in one try
すぐに分かる = understand immediately
声で分かる = tell by (someone’s) voice

What does 〜ように mean in 一目で分かるように?

〜ように after a verb often means:

“so that ~” / “in such a way that ~”

So:

一目で分かるように
= “so that (you) can tell at a glance”
= “in such a way that it’s understandable at a glance.”

Structure:

[clause in plain form] + ように
- 分かるように = so that (someone) can understand
- 聞こえるように = so that it can be heard
- 見えるように = so that it can be seen

Here, 自然が好きだと一目で分かるように means “so that it becomes possible to see at a glance that (I) like nature.”

What does 〜ようになっています express here?

〜ようになっています combines:

〜ように = “so that ~ / in such a way that ~”
なる = “to become”
〜なっている (te-iru) = “is in a state of having become ~ / has come to be ~”

So:

自然が好きだと一目で分かるようになっています。
= “It is set up / designed in such a way that you can see at a glance that I like nature.”
= “It has come to be such that you can tell at a glance that I like nature.”

This pattern is very common when describing settings, functions, or design:

ドアは自動で閉まるようになっています。
= The door is set up so that it closes automatically.
このボタンを押すと、電気が消えるようになっています。
= If you press this button, the lights are set to turn off.

It feels less like “I try to make it so” and more like “that’s how it is arranged / configured.”

What’s the difference between 分かるようになっています and 分かるようにしています?

Good contrast:

〜ようになっています
- Describes a state or design of something.
- “It is (now) in a condition such that ~.”
- Focus on the resulting setup, often not emphasizing the person’s ongoing effort.
Example:
- アイコンには森の写真を使っていて、自然が好きだと一目で分かるようになっています。
  → The icon is designed such that you can tell I like nature at a glance.
〜ようにしています
- Describes a habitual effort or intentional action by someone.
- “(I) make sure / try to ~.”
Example:
- 自然が好きだと一目で分かるようにしています。
  → I make sure (I set it up, I intentionally do things) so that you can tell I like nature at a glance.

In your sentence, the focus is on how the icon itself is set up, so 〜ようになっています is more natural.

Can you rewrite this sentence in simpler Japanese with the same meaning?

One simpler version, with two sentences:

アイコンには森の写真を使っています。自然が好きだと一目で分かります。

Meaning:
“For my icon, I use a photo of a forest. You can tell at a glance that I like nature.”

Your original sentence:

アイコンには森の写真を使っていて、自然が好きだと一目で分かるようになっています。

is a more connected, slightly more “explanatory” style, but the simpler two-sentence version is very natural and easier to parse.