And why?

Thkgk, sorry for not giving much detail in my last response.

It might be accepted but it is slightly 'odd' English. We usually talk about what's 'in' a picture rather than 'on' it. This is just a rule of the language and usage which as far as I know needs no justification. We can talk about someone painting something 'on' a picture but then this implies that it has almost been added after the fact. Think of a graffiti artist who paints a picture on the wall and then another graffiti artist comes along a week later and adds bits to it or paints his signature right across the middle of it. Then we would say these things had been painted 'on', because they were not part of the original composition or intention of the artist. Or think of someone painting a moustache on the Mona Lisa. The sentence 'The painter paints a farmer on the picture' has this kind of implication. I do not want to criticise anyone who is learning English and makes this kind of mistake. I'm only explaining this for the benefit of English learners. It is a very nuanced difference. And it's the kind of thing that would mark someone out who speaks otherwise impeccable English as a non-native speaker. No great misunderstanding would occur if someone talks about something been painted 'on' a picture, but it is unusual and sounds a bit strange. Hope that helps.