Click on the "User CP" in the upper left corner, then scroll down to "Edit Signature" and the rest is known
(I hope...)

You know yours is 270 x 123... right? It's the height that is usually an issue.That's annoying (don't mean to be rude but it is). Try not to exceed a size of 220 by 90 pixels.
agreedYou know yours is 270 x 123... right? It's the height that is usually an issue.
Also, the rule is 480x70.