YJ Pop Login appearance can be changed through css.
Personally i would make the wanted changes in the template.css file, this is to keep things tidy, and change background to something like:
background: url("../images/my_button.gif") no-repeat scroll left top transparent;
It's better to place your own image in the images folder of your template, and that is the path the url above is pointing to.
Please feel free to ask away if you get stuck anywhere with making these adjustements.