well, proportion is quite important in presentation, I tend to keep my banners small enough so that computers with smaller resolutions can still see it quite well.
I also like to use Ranger sprites if possible, they are often proportioned quite well. With the new ranger game, even more Pokemon have an awesome Ranger sprite. If you can save those sprites and somehow edit it, great. But if you don't, there is a few know hows to actually just use the image from serebii, as the page locks the right click button.
And you should pick back up your Sneasel, bring me an UT Eleactabuzz (as in the name, no NN).