1. We can see a person in the picture. The blues are on/at the background.

Question

(Which preposition do we have to use?)

2. By whom was this painting painted?
3. By whom was this picture painted?
4. By whom was this painting drawn?
5. By whom was this drawing drawn?
6. By whom was this drawing painted?
(Which ones are ungrammatical? Can we use the verb 'draw' with 'painting'? Can we use both 'draw' and 'paint' with 'picture'?)

Answer 1

If it's a painting, it's not a drawing. A picture, though, may be a photograph, a painting, a drawing...

Blues in the background: either in or on, not at.