Devo selezionare le foto migliori prima di inviarle.

Because after a modal verb like dovere (must / have to), Italian normally uses the infinitive of the main verb.

• devo = I must / I have to
• selezionare = to select

So:

• Devo selezionare = I have to select

This is the same pattern you see in:

• voglio mangiare = I want to eat
• posso andare = I can go
• devo studiare = I have to study

Devo is the first person singular of dovere.

So it means:

• I must
• I have to

In everyday use, dovere often expresses obligation or necessity.

Quick forms of dovere in the present tense:

• io devo = I must
• tu devi = you must
• lui/lei deve = he/she must
• noi dobbiamo = we must
• voi dovete = you must
• loro devono = they must

Because migliori is an adjective and it has to agree with foto.

• foto here is plural
• foto is treated as feminine in Italian
• so the adjective must also be feminine plural

That gives:

• le foto migliori = the best photos

A useful point: migliore is singular, and migliori is plural.

Examples:

• la foto migliore = the best photo
• le foto migliori = the best photos

Le is the feminine plural definite article, meaning the.

So:

• la foto = the photo
• le foto = the photos

Italian often uses the definite article where English also uses the, and sometimes where English might leave it out depending on context. In this sentence, le foto sounds natural and specific: the photos.

Migliore / migliori means better / best, depending on context.

Here, le foto migliori means the best photos or the better photos, depending on the situation. In natural English, the best photos is usually the most likely translation.

Why not più buone?

• buono means good, but with photos, Italian very commonly uses migliore to mean better/best in a general quality sense.

Why not più belle?

• belle means beautiful, which is more specific.
• migliori is broader: best overall, not just prettiest.

Prima di means before.

When the subject stays the same, Italian commonly uses:

• prima di + infinitive

So:

• prima di inviarle = before sending them / before I send them

This is very common:

• prima di uscire = before going out
• prima di parlare = before speaking
• prima di partire = before leaving

If the subject changes, Italian often uses a clause with prima che instead.

For example:

• Devo selezionare le foto prima che tu le invii.
• I have to select the photos before you send them.

Because Italian often attaches object pronouns to the end of an infinitive.

Here:

• inviare = to send
• le = them

So:

• inviare + le → inviarle

This is called attaching a clitic pronoun to the infinitive.

You could think of it as:

• prima di inviare le foto
• prima di inviarle

Both are possible, but inviarle is more compact and avoids repeating le foto.

Other examples:

• voglio comprarlo = I want to buy it
• devo farlo = I have to do it
• prima di leggerli = before reading them

Le refers back to le foto.

Since foto is:

• feminine
• plural

the direct object pronoun must also be feminine plural:

• la = it (feminine singular)
• le = them (feminine plural)

So:

• inviarle = to send them
• and them = the photos

Because it is a direct object pronoun, and it must match foto in gender and number.

For direct objects:

• lo = him / it (masculine singular)
• la = her / it (feminine singular)
• li = them (masculine plural)
• le = them (feminine plural)

Since foto is feminine plural, the correct pronoun is le.

Gli is different:

• it usually means to him or to them as an indirect object
• so it would not fit here

Here, the photos are the thing being sent directly, so a direct object pronoun is needed: le.

Yes, absolutely.

Both are correct:

• prima di inviare le foto = before sending the photos
• prima di inviarle = before sending them

The version with inviarle is more natural when the noun has already been mentioned and you do not want to repeat it.

Yes, there is a slight difference.

• selezionare = to select
• scegliere = to choose

Both can work in many contexts, but selezionare often sounds a bit more like picking items carefully from a group, which fits well with photos.

So:

• Devo selezionare le foto migliori = I have to select the best photos
• Devo scegliere le foto migliori = I have to choose the best photos

Both are understandable and natural, but selezionare can sound a bit more deliberate or technical.

In this sentence, inviarle means to send them.

Why? Because here -le is a direct object pronoun referring to le foto.

There is also an indirect pronoun le meaning to her, but context usually makes the meaning clear.

Compare:

• Devo inviarle. = I have to send them.
  or, in another context, I have to send to her is not complete unless something is understood.

• Devo inviarle le foto. = I have to send her the photos.
  Here le clearly means to her, because le foto is already the direct object.

So in your sentence, since le foto is what is being referred back to, inviarle means to send them.

Yes, but Italian also has some flexibility.

The given sentence:

• Devo selezionare le foto migliori prima di inviarle.

is natural and standard.

Its structure is:

1. Devo = I have to
2. selezionare = select
3. le foto migliori = the best photos
4. prima di inviarle = before sending them

You could rearrange parts in some contexts for emphasis, but the original order is the most neutral and natural for everyday Italian.

For a learner, this is a very good pattern to remember:

• [modal verb] + [infinitive] + [object] + prima di + [infinitive + pronoun]