Wir schneiden Bilder zu, es sei denn, sie sind beschädigt.

Because the verb here is the separable verb zuschneiden.

• Full infinitive: zuschneiden
• In a main clause, the prefix separates:
  • Wir schneiden Bilder zu.

This is a very common pattern in German:

• ankommen → Der Zug kommt an.
• aufmachen → Ich mache das Fenster auf.
• zuschneiden → Wir schneiden Bilder zu.

So schneiden ... zu is not two unrelated words; it is one verb split into two parts.

Here zuschneiden means something like:

• to cut to size
• to trim
• to crop

With Bilder, it often means to crop pictures/images so they fit a certain size or format.

So Wir schneiden Bilder zu is not just we cut pictures in a general sense. It suggests adjusting them by cutting/trimming them to the right size.

German often leaves out the article when speaking generally about things.

So:

• Wir schneiden Bilder zu = We crop pictures/images in general
• Wir schneiden die Bilder zu = We crop the pictures (specific ones already known)

Both are possible, but they mean slightly different things. Without die, it sounds more general or like a description of what we do.

Bilder is the plural of Bild.

In this sentence, it is the direct object of the verb zuschneiden, so it is in the accusative case. But for many plural nouns, the nominative plural and accusative plural look the same:

• singular: das Bild
• plural: die Bilder

So here Bilder simply means pictures/images.

Es sei denn is a fixed expression meaning:

• unless
• except if
• except when

So:

• Wir schneiden Bilder zu, es sei denn, sie sind beschädigt. = We crop pictures unless they are damaged.

It is best learned as a set phrase, because its form is a little unusual.

Sei is a subjunctive form of sein (specifically Konjunktiv I), but in es sei denn this is mostly just part of a fixed idiom.

So learners usually do best to remember:

• es sei denn = unless

You do not usually need to analyze it literally every time. In modern German, this phrase is extremely common and natural, even though sei may look unusual at first.

Because the clause after es sei denn normally has main-clause word order.

That means the conjugated verb stays in the usual second position:

• sie sind beschädigt

Compare this with a subordinating conjunction like weil, where the verb goes to the end:

• ..., weil sie beschädigt sind.

But with es sei denn, German usually keeps the normal finite-verb position:

• ..., es sei denn, sie sind beschädigt.

Sie refers to Bilder.

Since Bilder is plural, the pronoun is also plural:

• Bilder → sie
• the pictures/images → they

So sie sind beschädigt means they are damaged, where they = the pictures/images.

The commas separate the different parts of the sentence.

Structure:

• Wir schneiden Bilder zu,
• es sei denn,
• sie sind beschädigt.

German punctuation often marks off inserted clause-introducers like es sei denn. So the commas help show that the second part is an exception to the first part.

Here beschädigt is being used as an adjective, meaning damaged.

So:

• sie sind beschädigt = they are damaged

This is similar to English, where damaged can also function like an adjective.

It does not necessarily focus on how the damage happened. It just describes their condition.

Yes, there are other ways to express a similar idea, but es sei denn is a very natural and standard choice.

For example:

• Wir schneiden Bilder zu, wenn sie nicht beschädigt sind. = We crop pictures if they are not damaged.

This is close in meaning, but the structure is a little different: it states the positive condition instead of the exception.

So:

• es sei denn focuses on the exception: unless
• wenn ... nicht focuses on the condition: if ... not

Both can work, but they are not always identical in tone.

It can mean either, depending on context.

Bild / Bilder can mean:

• a picture
• a photograph
• an image

So Wir schneiden Bilder zu could refer to:

• trimming printed pictures
• cropping photos
• editing digital images

German leaves that open unless the context makes it specific.