Why does "je veux beaucoup de photos" mean "I want a lot of photos" and not "I very much want some photos" or "I want photos very much"?
In this spot "beaucoup" could be related to the verb or the complement. It can be tricky; but here, there is only one correct interpretation. If it was related to the verb, it would be optional:
- Je veux. Je veux beaucoup.
- I want. I want a lot.
Both make sense. But the complement does not make sense without "beaucoup":
- Qu'est ce que tu veux ? Une photo ? Deux photos ? Beaucoup de photos ?
- What do you want ? One photo ? Two photos ? A lot of photos ?
You could not remove "beaucoup", so it has to be related to "de photos". In the same way that "of photos" alone would mean nothing in English here.
I think you could think of it like the difference between "a lot of" and "a lot".
"Beaucoup de" means "a lot of":
Il y a beaucoup de sucre dans mon café = there is a lot of sugar in my coffee
Il a fait beaucoup de fautes dans son texte = he made a lot of mistakes in his text
"Beaucoup" means "a lot" (or "very much"):
J'ai beaucoup marché aujourd'hui = I walked a lot today
Je pense beaucoup à elle = I think a lot about her
Je l'aime beaucoup = I like her very much/a lot
Now with what I explained, you could think it's correct to say "Je veux beaucoup des photos" (with "des" for "some" instead of "de" for "of") to mean "I want photos very much", and the thing is: you can't but I couldn't explain why. You want to say "Je veux vraiment des photos".
It's true that technically, "beaucoup" should work. It's an adverb describing an intensity, why could you "aimer beaucoup quelque chose" but not "vouloir beaucoup quelque chose"? And note that I've heard many times things like "Je veux beaucoup aller à Paris", which sounds strange but not incorrect, so "vouloir beaucoup quelque chose" doesn't seem to be the real problem here.
I am no native speaker but I have been studying the French language for three years. To a beginner I would suggest that he/she learned set expressions first (also called fixed expressions). In the beginning just try to avoid to explain the constructions or render everything literally in your mother language. It is not often possible to translate word by word everything. For instance it is a most difficult task to render expressions like "est-ce que".
Je veux beaucoup de photos.
is interpreted by
I want a lot of photos.
I very much want some photos.
I wouldn't mind getting some photos.
one may say, respectively
Je veux vraiment des photos.
Je veux bien des photos.
Try to keep in mind the first paragraph of my response.