Learn Hardcore Indonesian: Setelah acara selesai, fotografer akan memilih gambar terbaik untuk dimasukkan ke dalam album. - After the event is over, the photographer will choose the best pictures to put into the album.

So acara selesai literally is event finished, but it is understood as the event is finished / the event has finished.
This is very common and correct in Indonesian. You do not need a word like adalah here.

What is the difference between Setelah acara selesai and Setelah acara berakhir?

Both are natural and very close in meaning.

Setelah acara selesai
Literally: after the event is finished.
Focus: the state of being finished/completed.
Setelah acara berakhir
Literally: after the event ends.
Focus: the action of ending.

In most contexts they are interchangeable. selesai feels a bit more like completed, while berakhir highlights the ending as an event.

Can I say Setelah selesai acara instead of Setelah acara selesai?

Setelah acara selesai is the most natural and standard.

Setelah selesai acara is not wrong grammatically, but it sounds less natural and a bit awkward.
A more natural alternative, if you want to move selesai, is Setelah selesai acara itu, but even then, most people still prefer Setelah acara selesai or Setelah acara berakhir.

For learners, stick with Setelah acara selesai or Setelah acara berakhir.

What does akan do in fotografer akan memilih? Is it necessary?

akan marks future or planned actions, similar to will or going to.

fotografer akan memilih = the photographer will choose / is going to choose

If the future time is already clear from context, akan can be omitted:

Setelah acara selesai, fotografer memilih gambar terbaik…
This can still be understood as a future-like sequence (after the event is over, the photographer chooses…).

Using akan makes the future/planned meaning clearer and is slightly more formal. In this sentence, it is very natural but not strictly required.

What is the difference between memilih and the base word pilih?

pilih is the root, and memilih is the active verb form.

pilih = choose (root form, used in dictionaries, word formation, some fixed phrases)
memilih = to choose, to select (used as a normal verb in sentences)

In a sentence like this, you should use memilih:

fotografer memilih gambar terbaik = the photographer chooses the best pictures

The bare root pilih would sound incomplete or telegraphic as the main verb in standard Indonesian here.

Why do we say gambar terbaik, not terbaik gambar?

In Indonesian, adjectives usually come after the noun they describe.

gambar terbaik = best pictures
(gambar = pictures, terbaik = best)

So the pattern is:

noun + adjective
e.g. rumah besar (big house), baju baru (new clothes), film menarik (interesting film)

Putting terbaik before gambar (terbaik gambar) would be ungrammatical.

What exactly does terbaik mean? Is it the same as paling baik?

terbaik is a superlative form made with the prefix ter-, meaning the best.

baik = good
paling baik = the most good / the best
terbaik = the best

In most contexts, paling baik and terbaik are interchangeable. terbaik is shorter, very common, and sounds natural and neutral here:

gambar terbaik = the best pictures

Why is dimasukkan used instead of memasukkan? Why the passive voice?

memasukkan = to put something into (active voice)
dimasukkan = to be put into (passive voice)

untuk dimasukkan ke dalam album literally: to be put into the album.

Indonesian often uses the passive when:

the object is more important than the subject, or
the subject is obvious or already known.

Here, the focus is on the pictures being selected to be put into the album, not on explicitly repeating the photographer again. Passive makes the sentence smoother and more natural:

Active version: … fotografer akan memilih gambar terbaik untuk memasukkan (gambar-gambar itu) ke dalam album.
Passive version (original): … fotografer akan memilih gambar terbaik untuk dimasukkan ke dalam album.

The passive is preferred here.

How is dimasukkan built, and what does -kan add?

dimasukkan is formed from:

prefix di- (passive)
root masuk (to enter / go in)
suffix -kan (causative / to put something somewhere)

So:

memasukkan = to cause something to go in → to put something into
dimasukkan = to be caused to go in → to be put into

Here, dimasukkan ke dalam album = to be put into the album.
The suffix -kan indicates that something is being put into a place, not just entering by itself.

Why is it ke dalam album, and not just ke album or dalam album?

All three can be used, but there are slight differences:

ke dalam album
Literally to inside the album – emphasizes movement into the inside of the album. Very natural with verbs of putting/placing.
ke album
Literally to the album – focuses on destination, less on the idea of going inside; still acceptable.
dalam album
Literally in the album – describes location (where something is), not movement into it.

Since dimasukkan is about putting into, ke dalam album matches that idea very well and sounds very natural.

Is ke dalam always used together like this, or are ke and dalam separate words?

ke and dalam are separate prepositions:

ke = to / towards (movement)
dalam = in / inside (location or the inside of something)

When used together as ke dalam, they combine the ideas: to the inside of.
You will see this pattern often with verbs of entering or putting something in:

masuk ke dalam rumah = go into the house
menaruh buku ke dalam tas = put the book into the bag
dimasukkan ke dalam album = be put into the album

Why is there a comma after selesai: Setelah acara selesai, fotografer…? Is it required?

The comma separates the introductory time clause from the main clause.

Setelah acara selesai = time clause (After the event is finished)
fotografer akan memilih… = main clause

In good written Indonesian, a comma is normally used after an initial clause like this. In informal writing, people sometimes skip it, but including the comma is recommended and standard:

Setelah acara selesai, fotografer akan memilih gambar terbaik… ✔ (preferred)

Can I use other words instead of fotografer, like tukang foto or juru foto?

Yes, but the tone changes:

fotografer
Neutral, standard, can be professional-sounding. Appropriate in most contexts.
tukang foto
More informal/colloquial. Can suggest someone whose job is taking photos, maybe less formal or more street-level (e.g. a photo booth person).
juru foto
Sounds a bit old-fashioned or formal; still understood.

In this sentence, fotografer is the most natural and neutral choice.