If you're an artist, create vectors--they sell well and if they are sufficiently detailed, some microstock sites will tag them as more expensive downloads--meaning more money to you. Selling vector images at Shutterstock means working within the (essentially) all-you-can-eat package--which can also mean many, many downloads. IStock rates each vector based on degree of difficulty and puts them into different price categories based on that. You can use Illustrator, Freehand, or free software such as Inkscape to create your imagery.