Yes, MemberGate is responsive so it will decrease the size of any images to fit the screensize of the user. This means a large image will shrink to fit in the smaller screen, but in a larger screen that image might be too large.
MemberGate includes a built in image editor so some simple editing can be completed right from the page that is being edited.
*BONUS - Changing the size of the image will also decrease the file size of the image, which makes for faster page loading times.
Image Editing Tools Available