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Are you doing all the usual things, like uploading it to an image site, copying the url, and adding the link to your Signaure, and surrounding it with [IMG] tags?
"There are you, you drive like a demon from station to station," - David Bowie
Which image host are you using? Some of them change the file type around which makes it trip out sometimes (I.E the URL says JPG and the image is a PNG /GIF)
When I try to use the image you sent me in my sig, it says "Your images may only be up to 350 pixels high."
This means you need to resize it. Where does it say that it can't determine the image dimensions for you? I'm not able to replicate this error when I copy and paste the code I sent you yesterday via PM into my sig.