Staying on the subject of stock photography, paintings and illustrations also sell well on these sites.
This is one of my acrylic pours. The actual size is very large, 3 x 4 feet. I went to youtube.com and learned how to mix a combination of acrylic paint, pouring mediums, treadmill oil and water to make this beautiful painting. After pouring I used a torch and the cells come through. Check with the artists who do this type of art for specific details on this type of artwork. It may take some practice to get the materials just right. Very fun but very messy! This one does sell on my stock photography sites. As I have mentioned before this will need a model release, yes, for your own painting. Take a photo of it before it is completed with the surroundings showing and of course a very good photo of the original art for selling digital images.