There are a few things:
1) There is a horizontal line/cutoff at the very top of the render
2) The first one is of better quality than the second
3) There is a lot of empty space. I would maybe do the sig at 450x150
4) Maybe make the red background as dark as the red on her hat or shirt
but if this is your first sig, you've got a decent base to start from...so just keep practicing.