In the sentence 'She is happy', 'happy' is a predicative adjective.
You can change the adjective 'sweet' into a noun by adding the suffix '-ness' to the end of the word.
In 'a sudden movement', 'sudden' is an adjective in the attributive position.
I don't think I'd call it a beautiful picture - 'interesting' might be a better adjective to use!
However, this asymmetrical distribution means that the only context that permits comparison among really, very, so and pretty as intensifiers is the predicative adjective context.
By contrast, ordinary adjectives without passive morphology were unlikely to be reanalyzed as verbs or to give rise to a verbal interpretation.
This could be due to a purely syntactic difference between number words and adjectives.
