This looks decent. The only concern I have about this image is that unlike Vaporeon and Flareon, Jolteon just seems to have an angry look in its face as if something had just caught its attention and made it very angry. That and there is some white on the edge of its eye too.
The background itself is nice. The polka dots and stripes make a nice combination for the original Eeveeloution trio.
Overall, aside from an eye error on Jolteon's face, this looks good for a wallpaper.