Without seeing your portfolio, it's a bit of a guess, but I would say that you should easily be able to beat that income by uploading elsewhere. Look at the income numbers on the right and see where DT ranks in the top 4 - they just can't break out of that "bottom of the top tier" slot that they've held since I first started with them (in 2004).
You don't say if you do vectors or raster illustrations, but every agency except iStock will be fine with raster illustrations; iStock makes it hard to get raster illustrations accepted and refuses JPEGs of vector-created art. They are OK with scans or photos of paintings, watercolors and other "real world" media.