If you click on the little circle with an "i" inside you can always view the recommended image sizes.
Logo: 214 x 24
Background: 1200 x 800
Recommended size for images for your Section tab: 380 pixels wide by 225 pixels tall.
For the three column section option: It's important to make sure all images are the same height. Otherwise, the alignment of the columns may get thrown off.
JPG or PNG files are accepted.