jungyohan sajineun keompyuteo-edo baegeophae duseyo.

중요한 means important.

The dictionary form is 중요하다 (to be important).
When a descriptive verb/adjective like 중요하다 directly modifies a noun, it changes form:

• 중요하다 = to be important
• 중요한 사진 = important photos / important pictures

So 중요한 사진 literally means important photos.

은 is the topic marker, while 을/를 is the usual object marker.

So:

• 사진을 백업해 두세요 = Back up the photos.
• 사진은 백업해 두세요 = As for the photos, please back them up.

Using 은 gives a slight sense of topic, contrast, or special attention. In this sentence, it sounds like:

• At least for important photos, make sure you back them up
• or Important photos, please back those up

So even though photos are still the thing being backed up, Korean often uses 은/는 to highlight them as the topic.

컴퓨터에도 is made of:

• 컴퓨터 = computer
• 에 = to / on / in (depending on context)
• 도 = also / too

So 컴퓨터에도 means to the computer too or on the computer too.

The idea is that the photos should be backed up to a computer as well, probably in addition to somewhere else, such as:

• a phone
• cloud storage
• an external drive
• a memory card

도 means also/too, so it implies there is another location or method already being considered.

The sentence does not explicitly say what that other place is. Korean often leaves that unstated if it is clear from context.

For example, the speaker may mean:

• Don’t keep them only on your phone; back them up on your computer too
• Save them to the cloud, and to your computer too

So 도 adds the nuance of in addition.

백업해 두세요 is more than just back it up.

It breaks down like this:

• 백업하다 = to back up
• 백업해 = do the backup
• 두세요 from 두다 = do something and leave it done / do it in advance for later

So 백업해 두세요 means something like:

• Please back them up and keep them backed up
• Please go ahead and back them up
• Please make sure they are backed up

Compared with 백업하세요, 백업해 두세요 has a stronger feeling of:

• doing it in preparation
• doing it before it becomes a problem
• leaving the result in place

It is very natural in advice or warnings.

Not by itself. 백업 is a loanword from English backup.

In Korean, many loanwords become verbs by adding 하다:

• 백업하다 = to back up
• 다운로드하다 = to download
• 체크하다 = to check

In this sentence, 백업하다 becomes 백업해 before 두세요.

세요 makes the sentence a polite request/instruction.

So 백업해 두세요 is a polite way to say:

• Please back them up
• Make sure you back them up

It is softer and more courteous than a plain command.

It can mean either photo or photos, because Korean often does not mark plural unless it is necessary.

So 사진 can mean:

• photo
• photos
• picture
• pictures

In this sentence, English would most naturally translate it as important photos, but the Korean itself does not force plural marking.

Korean often leaves out subjects when they are obvious from context.

Since this is a polite instruction ending in -세요, the listener is understood automatically. So the omitted subject is basically:

• you

A more literal English version would be:

• You, please back up important photos on your computer too

But Korean normally does not need to say you here.

Technically, 도 can sometimes feel like even, depending on context. But in this sentence, also/too is the most natural reading.

So the most natural interpretation is:

• Back up important photos on your computer too

If the speaker wanted a stronger even nuance, the sentence would usually need context that makes the computer seem surprising or unexpected.

The sentence sounds like practical advice or a reminder.

A natural English sense is:

• Please back up important photos to your computer too
• Make sure you back up important photos on your computer as well

The key nuance comes from -아/어 두세요: it suggests doing this ahead of time, so the photos stay safe later.