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Some rules regarding signatures...
15. Signature images may not be larger than 600 pixels wide by 125 pixels tall. This includes logos, photos and other graphics. Also, no more than one row of images are allowed. If you want four or five logos in your signature, reduce their size to fit in one row.
16. Limit the size of your photos to a maximum of 800 pixels wide. (this does not include signature images - see rule #15 above)
Use his link to resize it if you need to - http://www.simpleimageresizer.com/