I don't do a lot of illustrations but I am thinking of doing more. I have used the free program inkscape to do a few simple vectors
Inkscape. Draw Freely.
They have all sold well, so during the winter months I will try and do more. I do a few backgrounds with photoshop but they don't sell as well. SS and DT seem like the best sites for simple illustrations. I was turned down by istock but will try again when I have some more experience. istock also reject all my photoshop backgrounds.