It's frustrating to be told to provide graphics but not be told what dimensions the graphics should be. User pics are square, so that's easy, but I'd like something during the uploading cover photo progression to indicate what size the rectangle needs to be.
I didn't really notice the issue when I uploaded the picture for my user cover photo because I used a landscape and it didn't really matter where the image was cropped. But when I created a new community yesterday, I wanted to create a cover photo with writing on it and it took several tries to get the image the exact right ratio so that all of the writing was visible when I uploaded it. And the first time I did that, it looked fine on the upload dialog but I still had to re-edit because when I went to the community site to make sure everything looked right before I told people I'd made it, I discovered that the image I'd provided had actually been further cropped and one of my lines of text was cut in half. It's all good now (in part because I went with something more simple), but I feel the process would have been easier if I had been told "Provide an image with an x/y ratio and z amount of optional space."
Also... I don't know how the cover images will work once we have a web browser interface. On many networks, the size displayed is different depending on which device you view on. If this will be true on Openbook, I would really appreciate being able to have different images for in-app and web browser display.