Recommended size for background/profile image
For normal template which uses the picture as something like a profile picture, we have a file size limit of 8MB and an image size limit of 1200 pixels wide. Though you may be able to upload larger pictures, but the profile picture will still remain at the size of 340x240.
The template which places your photo as a background image has a recommended size of 1920x1080
Center-wide main image is 960x300.
NOTE: If you picture is larger than the recommended size, the system will automatically resize it and make it smaller to fit the recommended size. If the picture has a smaller size than what is recommended, then the system will also resize it but makes the picture blurry as it will zoom in for the picture to fit the space.