The thumbnail size is a very important factor to consider in your microstock strategy.
If you look at your images at the thumbnail size, compared with all the others on the page, you will guess at once if your image is going to be a good seller.
It stands out? The main subject is easily recognizable? The colors are saturated and bright? Simply answering to yourself those questions may improve a lot your sales.
For more about this topic and other beginners advices on microstock, you can also read my personal blog, that I recently started at: saniphoto.blogspot.com