Because these show up on every post you make there are rules to using this because....
1).dimensionally large pictures can distort the forum for users and...
2).large images with a lot of bytes/information can take a long time for our dial-up users to download and see....50% of users on the internet are dial-up users; please show courtesy and don't post monster pictures

For a signature to be within limits it must be within
max physical image size: 400X400 (width x height)
max 'space' image size:55,000 bytes
To check your signature size while using Internet Explorer for a browser...
1).right click on the photo.
2).left click on properties
Size must be 55,000 bytes or less
Dimension must be 400 X 400 or less. Can also be 400 X 400 or less.
If you have multiple images the total is added where for example, if you have a 400 X 100 picture and a 400 X 150 picture, one above the other, in your signature it would be considered 400 X 350 in total. The width is not being added to but the height is because they are above each other. If these two pictures were side by side it would not be allowed for it would be 800 X 150 in total.
If you find that your signature is not working first check to make sure you are actually adding it. In your profile go to your profile tab->click on profile settings on the left->click on posting->make sure "yes" is checked next to Always attach my signature.
If the above is ok, your signature has probably been turned off by an administrator. It's current policy to not notify users when their signature has been turned off for notices concerning signature sizes have been posted for 2+ yrs. now. If yours is off, check you signature size per the rules above, fix it, and then let an administrator know so it can be turned back on.
If you don't know how to fix it...you should

Also, one of the main reasons there is a signature size limit on width is because of it creating a horizontal scrollbar for some users. What can also cause this in normal posting or signatures is long lines of text without a break/space...or normal length text such as a url address that is formatted to use large fonts. Though it doesn't happen often, please be aware that this can occur and I watch for these, too.
thanks,
Steve