What do I tell the user if the image they have to upload is very specific in height and width?

Am making an android app. I need to make the user be able to upload an image of a button that is pre-configured in size. So they would essentially be only changing the design.

My questions:
1. Should I limit the size and reject any image that is not of exact size in pixels (807x185) - OR - not reject anything but make the user at fault if the button looks bad/distorted? (basically do I make the button and the image the same exact size or mold and fit the image into the 807x185 button frame no matter if it's smaller or bigger)
2. How should I tell the user about this limitation?