I think it's an okay size. You can go up quite a bit to 100x100 though. It depends on how many you want to put in your signature. Most sites have restrictions and bigger images means less room in sigs and more and less advertisement, depending.
I like the idea of instead of eggs, having flares. Um, like these:
http://fc03.deviantart.net/fs70/i/20..._by_manj3d.jpg
(Only multi-colored and maybe you can see the vague shape of the adoptable in it.)