How to discourage uploading image with text

How can I discourage the user from uploading a header image with text in it? It's explicitly mentioned in the preceding description (in red, as displayed below) and in the tips panel to the right, but people keep on uploading images with text in it (example image shown below). What would be a better way of telling the user not to upload images with text?

The reason for disallowing text, is that the image will be used as a background image, so a title will be centered on top of the image.

Displayed above is a user who uploaded a header image with text.