Hi Jeffrey,
The thumbnail picture we will need to resize as a square, but the most important thing is that all your photos are the same size so when people browse the photos, the size isn't jumping from different dimensions. Although if it has to be it is also fine. As for the photos, we usually suggest submitting 5 photos from different angles of the mod, along with zoomed in shots, and the main one is usually the one showing most of the mod, front angled and showing the side.
It will show similar to the way the gallery is here http://makerhub.coolermaster.com/modding-gallery
If you have a specific request please email mod@coolermaster.com.tw