
I have the grid allocated to 25 slots since it sounds more and more like that will be the case, but just using 20 works fine too. 25 for sure now.

The individual boxes are 100x100 pixels, which match the size of the thumbnail pet pictures on this page (Gallery of Pets). Simply copy the image from that page and paste it into a box using an image program (MS Paint suffices just fine).
Empty grid: