On iPhone and iPad, optimum size would be 400px width x 320px height. The picture’s with dimensions greater than these will be scaled to fit.
For other mobile devices, the actual image will be scaled to fit that devices screen size. So a good approach is to use the 400px width x 320px height dimension.
Supported formats are png, jpeg, and gif.
Check your offer before you publish it just to make sure the picture looks the way you want.