I'm sorry you are sad, Betsy.
I can relate to the feeling. During one of our break-ups, I gave my exbf a necklace of mine--just a little siler necklace with a moonstone dangling as its pendant. I gave to him because it was really special to me and because he would always say that I was just like the full moon: beautiful and enchanting.

Plus, I would always tell him that he was my half-moon, and he would respond saying I was his--thus, together we made the full moon, which we agreed, was the most marvelous of the moon phases--so full of love and warmth.
I gave him this so he would "remember me." Last time I was at his place, he still had it hung ontop of his bed. Seeing that made me wonder the same as you: Does he still care for me? Is this why he still has it?
But you know, what does it matter? The truth is that you'll never know, anyway, unless he someday tells you. Maybe he does still care or maybe he has it there because he likes it or maybe he's just too lazy to adorn the place with another thing or maybe he doesn't even remember it's there or the significance of it!
It's sad, but it's true.
I personally think he still has your painting because yes, he still harbors feelings towards you, and I am sure those feelings are care and love. But please realize that this may no longer be the same type of love--maybe he loves you as a person, and no longer as a lover. Regardless, I am sure he still keeps such a personal piece because of the feeling behind it and the care he still has for you.
It may not be the same and it may not be in the same amount or in the same way you wouldn't want it to be, but know that he most probably still does care for you, in one way or another.
Knowing that should be enough, and if it's not, the pretend you he threw it away.