When uploading a Header or Footer image, the dimensions must be 990px by 210px
You can quickly resize your image to fit these dimensions by going to:
http://resizeimage.net/
Upload the image you are wanting to resize for the header or footer
2. Uncheck the box for "Keep Aspect Ratio" in the Resize Your Image section
3. Change the image size to 990 x 210px
4. At bottom of page, click to "Resize Image"
It then gives you the option to View or Download the new image