I'm trying to submit a layout but I keep getting a message that the screenshot must be 4:3 or widescreen. I have tried all of the recommended sizes, but it still isn't going through. Any suggestions?

This is the image I'm using: Mummy Screenshot

Your problem is that createblog can only use screenshots that are a certain ratio. 4:3 and widescreen. Which means that 1280px × 768px is not a valid ratio for createblog. You can resize the width of the image to 1024px and it will be a 4:3 ratio. The easiest way to make sure that you are using the correct size images is to take full screen photos and to NOT crop them, unless you are cropping them to a smaller 4:3 or widescreen ratio.

1280x768 is widescreen, isn't it?

Your resolution is too small. Try submitting the layout using this screenshot: