To create a network, you need a profile image (1) and a hero image (2). You can always change these images after you create the network. PNG, JPG, JPEG are all supported.
Profile Image Specifications
Square images that are 300 x 300 are ideal, though you can upload larger / smaller photos.
Hero Image Specifications
Images that are roughly 700 x 140 are great. Note that each screen size will crop the hero image differently. Images that can be cropped in different sizes are best.