The white space is part of the image itself it looks like o.o
Crop some of the white space out using an editing program.
There's about an inch and a half of white space on all sides of the centered image.
I did a quick crop on it for ya, but it washed out some of the colors a bit. You might want to ask the artist to crop it from their source file.
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If this is fine though, download from my link and host it from your preferred site.
Otherwise, make sure you don't have unnecessary line breaks in your signature's format.