Did you know that kissu's UI (and the old vichan version) supports a mascot system similar to the old oneechan extension that people used for 4chan?
If you go to [Options] and then 'Style', you can specify an image url to spice up your browsing experience with a cute girl or something else.

There's a couple things to note, however.
First, mascots should definitely have a transparent background. It's weird if there's a big solid block of another color. Maybe your tolerance is better than mine, however.
Secondly, you'll usually you want some kind of semi-transparency. My default for CSS stuff is 50%, but your preference may vary. I'll demonstrate this with two version of the same cleaned image.

This is the first image