Thumbnail images, URL, and/or project files can be attached for clients to view. Images can be in .jpg, .jpeg, .gif, or .png format. The minimum dimensions for images are 400px width x 300px height , however 1000px width x 750px height is recommended. Attachments can be in .doc, .swf, or .pdf format. The maximum size is 5 MB for the thumbnail and 25 MB for attachments. You might want to also check out this post for more information.