The resolution of a picture is dictated by the number of pixels that it has. The more pixels, the better the resolution of the image. However, the more pixels a picture has, the larger the file size.
To prevent the files that you send being excessively heavy and taking too long to send, the resolution of the photos we accept has been limited to a maximum of 3 Mega pixels, which is equivalent to an image size of 2000x1500.
The majority of digital cameras, as well as mobile devices in camera photo mode (except iPhone), have a menu of options where you can choose the resolution that you want to use. In this case, you must select a resolution equal to or less than 3 Mega pixels, or an image size less than or equal to 2000x1500.