Well I'm not sure whether you mean adding text onto a different sig, or replacing mine with your name.
(This is for Photoshop btw, not sure about GIMP)
For adding text to a different sig;
Click on the text tool. Click anywhere on the sig. Type out something :icon_razz:
Usually the text looks better with a 1px black border around it. To do this, go to the "Layers" window (if you don't see it, go to Windows > Layers) and right click on the Text layer you just added, then click on Blending Options. At the bottom should be "Stroke". Tick the box and click on Stroke. Change it to 1 or 2 px, and set it to any colour you want :lol:
For replacing the words on my sig you'd either have to get the .psd from me (in which case you could easily change the text) or use the healing brush over my name, which doesn't always work well :icon_wink:
Hard to explain with text