Learn Croatian: Ona ne želi previše slika. - She does not want too many pictures.

It is spelled exactly the same, but this is actually the genitive plural! Words that express quantity, like previše (too many), require the following noun to be in the genitive case. Because there are many pictures, we use the genitive plural. For most regular feminine nouns ending in -a, the genitive plural ending is also -a.

If the singular and the genitive plural look identical, how do I know which one it is?

You rely on the context! Here, the word previše (too much/too many) gives it away. It forces the noun into the genitive case, and since you can count pictures, you logically know it must mean "too many pictures" rather than "too much of one picture".

Could I use neće instead of ne želi here to say "she doesn't want"?

You could, but the tone shifts slightly. Ne želi (from the verb željeti) is a standard, polite way to say "does not want" or "does not wish for." Neće (the negative of htjeti) is stronger and can sometimes sound like a stubborn refusal, closer to "she refuses" or "she absolutely will not."

Why are ne and želi written as two separate words?