You're not being dumb, don't worry! I know exactly what you're talking about; there are times when my signature does the exact same thing you described and I believe it's due to the small size of the sig image coupled with a short post. If you look at this post of mine, it's got space at the bottom because my post is only one line of text, whereas it looks normal in this post. There's not much that can be done about it other than to make a longer post, but your signature is well within the limits so no worries on that front. Hopefully that makes sense!