The dimensions are fine, but the file size is a bit too big (32,180 bytes; only 30,000 are allowed). Could you downsize it by slightly reducing the DPI (dots per inch)? This freeware is easy to use: www.irfanview.com . If you need help, just PM me, another moderator, or mentor.
Never mind - I just went ahead and resized it. Could you copy and use this version? It's well within the size limits:
Fixed! If you use Irfanview, all you have to do is open your file, go to "Image", and then "Resize/Resample". The normal DPI is 72; by reducing it to 71 or 70, you'll reduce the file size without losing much picture quality. I love your background, BTW. Be careful - it's so easy to become addicted to making new siggies!
The allowed height is 150 pixels. You could try remaking your siggy at about 140 x 450 pixels, and then if the file size is too big, reducing the DPI by a notch or two.