No pongas la foto hasta que la pared se seque.

Because this is a negative command addressed to tú.

In Spanish, negative commands for tú use the present subjunctive:

• poner → yo pongo
• from that, the subjunctive is ponga, pongas, ponga, pongamos, pongáis, pongan

So:

• Pon la foto = Put up the photo.
• No pongas la foto = Don’t put up the photo.

No pones would mean you do not put or you aren’t putting, not a command.

No pongas is informal singular, used with tú.

Other versions would be:

• No pongas la foto = don’t put up the photo. (informal, one person)
• No ponga la foto = don’t put up the photo. (formal, one person)
• No pongáis la foto = don’t put up the photo. (informal plural, common in Spain)
• No pongan la foto = don’t put up the photo. (formal plural / standard plural in many regions)

Since this is Spanish from Spain, pongáis is especially worth noticing for vosotros.

Hasta que means until when it is followed by a clause with a verb.

Compare:

• hasta mañana = until tomorrow
• hasta las cinco = until five o’clock
• hasta que llegue = until he/she arrives

In your sentence:

• hasta que la pared se seque = until the wall dries

So hasta on its own is often followed by a noun or time expression, while hasta que is used before a full clause.

Because after hasta que, Spanish often uses the subjunctive when the action is in the future or has not happened yet.

Here, the wall is not dry yet, so Spanish treats that as something pending:

• No pongas la foto hasta que la pared se seque.
  = Don’t put up the photo until the wall dries.

If it were talking about something habitual or already completed in the past, Spanish could use the indicative instead in other contexts. For example:

• Esperé hasta que la pared se secó.
  = I waited until the wall dried.

So in your sentence, se seque is subjunctive because the drying is still unreal/pending from the speaker’s point of view.

Because they are doing different jobs.

• The first se is the pronoun from the verb secarse.
• seque is the subjunctive form of secar.

So this is:

• secarse = to dry / to become dry
• que la pared se seque = that the wall dries / becomes dry

It is not two pronouns. It is simply:

• pronoun: se
• verb form: seque

It just happens that they sound similar next to each other.

Because the wall is the thing that becomes dry by itself.

Spanish often uses the pronominal form secarse for to dry / to become dry:

• La pared se seca. = The wall dries / is drying.
• La pintura se seca rápido. = The paint dries quickly.

But secar without se is usually transitive, meaning to dry something:

• Seco la pared. = I dry the wall.
• Seca el pelo. = Dry your hair.

So:

• secar = to dry something
• secarse = to get dry / become dry

Because the wall is what is doing the action of becoming dry.

In English, we also say:

• the wall dries
• the paint dries

Spanish does the same:

• la pared se seca
• la pared se seque

So la pared is the subject, and se seque is what happens to it.

Spanish could use either one, depending on context.

Your sentence says:

• No pongas la foto... = Don’t put up the photo...

This is natural if the speaker wants to name the object clearly.

But if the photo has already been mentioned, Spanish could say:

• No la pongas hasta que la pared se seque.
  = Don’t put it up until the wall dries.

So using la foto here is just the full noun phrase. It is not grammatically required; it is a choice based on context and clarity.

Literally, yes: poner means to put.

But in real use, poner la foto can mean something more natural in English, such as:

• put up the photo
• hang the photo
• place the photo

It depends on context. If you are talking about a wall, English often prefers put up or hang.

Spanish uses poner very broadly, where English often chooses a more specific verb.

Yes. Colgar la foto means to hang the photo and is more specific.

Compare:

• poner la foto = put up / place the photo
• colgar la foto = hang the photo

If the photo is definitely going on the wall, colgar is very natural.
If the speaker is being more general, poner also works.

So this sentence is correct, but another natural version could be:

• No cuelgues la foto hasta que la pared se seque.

Because pared is a feminine noun in Spanish:

• la pared
• una pared

There is no special reason visible from the meaning; noun gender often just has to be learned with the word.

A good habit is to learn nouns with their article:

• la foto
• la pared
• el cuadro
• el muro

That makes agreement much easier later.

Yes, in normal modern usage, foto is feminine:

• la foto
• una foto

That is because foto is a shortened form of fotografía, which is also feminine:

• la fotografía

So even though the word ends in -o, it is feminine.

Yes, but it depends on whether the action is seen as pending/future or completed/real.

Use subjunctive when the action has not happened yet:

• Espera hasta que llegue. = Wait until he arrives.
• No pongas la foto hasta que la pared se seque.

Use indicative when referring to something that did happen:

• Esperé hasta que llegó. = I waited until he arrived.
• No puse la foto hasta que la pared se secó. = I didn’t put up the photo until the wall dried.

So the choice depends on time and viewpoint, not just on hasta que by itself.

It could be, but it would say something slightly different.

• hasta que la pared se seque = until the wall dries / becomes dry
• hasta que la pared esté seca = until the wall is dry

Both are natural and very similar in meaning.

The first focuses more on the process/result of drying.
The second focuses more on the state of being dry.

In everyday use, both can work well.

It breaks down like this:

• No pongas la foto = main clause, a negative command to tú
• hasta que la pared se seque = time clause, meaning until the wall dries

So the structure is:

• negative command
  • until-clause

A simple formula is:

• No + present subjunctive
• hasta que + subjunctive (when the action is still in the future/pending)

That is exactly what you see here.