@fanowll yup, that's allowed. If by chance the site resizes your image, that means it's too big and needs smaller file size, otherwise it's all good.
@drkwyr Your concept sheet has to include the front and back of your final design. Everything else is up to you to decide whether it's important enough to be on the sheet or not. So if you have too many sketches to fit, you don't have to include all of them. Use the space to present your work in the best way possible 