From the page where you can edit signatures...
Note: The maximum size of your custom image is 500 by 100 pixels or 19.5 KB
(whichever is smaller).
Blue, your image is 512 x 117 pixels and 19.9 KB. I'm guessing it's slightly too big size wise.
Wendy, your image is 327 x 193 pixels and 13.2 KB. It might be the height?
If you question BeakersBro, well, he cheated! Matt has multiple images in his signature, rather than one image... all strategically placed to create a larger image. If he's given a larger image of your signatures, he'd probably be willing to regurgitate the code back to have a signature image like his! (if I understood correctly from his post)
BTW, PNG (Portal Network Graphics) images are typically better than JPG or GIF images when you upload them to the internet. Large PNG images are typically clearer and sharper than the same images when uploaded in JPG or GIF format. GIF images also tend to be rather large. At least, this is what I've noticed in the past.