guidelines = instructions or rules for doing something: - The guidelines for uploading a photo are: JPG or PNG, less than 5MB - The installation guidelines are difficult to understand. guide lines = lines drawn on/over something to help align it, or the ropes/cables used to help people walk on a difficult path: - You can use the guide lines to help straighten the photo. - The guide lines will keep you from falling down the mountain. - The guide line will help you find your way in the dar.

I guess it means, in the case of italki, the pictures need to be a certain resolution of pixels, not exceed a maximum size, and the picture itself should be clear and appropriate.