Logo: 243 px wide by 54 px high
The logo can be either a jpg or png file, but the png will look the best.
Home page cover photo: 1920 px wide by 600 px high
Remember, the login box will be in the middle of that image so don't use text or have your face in the middle of that image.
Product image: 1920 px wide x 600 pixels (or taller)
This one product image will be seen in two different place (and will be two different sizes). The product image you upload will be seen on the product tile and the product page. Make sure your image is at least 1920 px wide and anything taller than 600 px will be cropped to 600 px for the product page.
Media Library Thumbnails: (for videos, audios, documents): 640x high by 360px wide