Make sure you use a large and clear main profile photo - the first impression is everything. If its width is smaller than 310 pixels, the system will enlarge it to fill the frame and it will look blurred and distorted.
Avoid having collages in your portfolio, especially vertical ones. This is a very common mistake that many users make. A portfolio is a set of photos and in a way it is a collage on its own. So, uploading a collage of photos, as one image, does not really compliment your book and your portfolio in general. If you really have to use a collage, try to at least limit the number of images in it to 2 and have them horizontally placed instead of vertically.
Photographers, don't place huge logos in your photos - this would only annoy your possible clients. Just open any magazine and see if any of the images there have anything like this on them. Even online, if you Google a famous photographer you will never find an image of theirs with a huge copyright logo across it. And yet most amateur photographers do the total opposite. Be smart and only do what the best do.