Given I don't know, my first thought was... well, give it a try and find out!

(But maybe someone else knows directly.)
I'm assuming you're concerned about the image looks when laid out and not that you necessarily want to upload large images. So... take the images you want and drop them in. If they come out too large, make them smaller until they look right to you. Ultimately, "optimal" depends on what you're trying to do. I would think the images should be sized the size you want them to be. Otherwise, you could take up needless bandwidth downloading more data than you need.