I'm sure this has been covered before but help would be appreciated.

I have been trying to add a 'picture' which someone kindly made for me to my signature.I have it stored in a file in 'my documents'.I tried to add it using the browse feature below the signature box, but it just says it is 'unable to save image'.

I tried to 'insert image' within the signature box by cutting and pasting ...and although the image appeared and I saved it,my signature now shows a little box with a red cross,so obviously I've done something wrong.
Can you point out the error of my ways?
Thanking you in advance