Well, it's just that it's plain.
I've said before in some other threads, that, Anime is NOT my strong suit, and because of it being so plain, I really can't give any advice.
But I do have a pro-tip for you.
After your sig is finished, make a new layer.
Ctrl+A will "Select All" and make a dotted line in a shape of a box around your image. Then goto Edit>Stroke
For now, use 1 pixel borders, and depending on the image use white or black until you get better.