(A 'tag' is the same thing as a 'sig' or 'signature' in case there was any misunderstaning)
His question is being misinterpreted. What he means, is when he places his render in to create a tag, he places it in, and grabs the the little circle resize buttons the the edge of the image and drags. When he is finished, the size is all distorted unless he had dragged it perfectly. Here is my recommendation for adding renders to a tag:
-Open your render.
-Depenging on your height/width of your tag, goto Image>Resize and change the height/width values. Make sure 'keep aspect ratio' is checked. When using a 320x100 template for my tag, I normally resize the width to 300, and let the aspect ratio take care of the height of the render.
-Select all by hitting Ctrl+A.
-Copy by hitting Ctrl+C.
-Goto your tag document, and hit Ctrl+V to paste in your render.
-When the prompt comes up asking you to resize your image, hit no.
And there you go, your renders should be inserted perfectly.
I did make a vid tutorial on this, so if you still can't get it, give me your e-mail and I will send you .AVI file.