There is a bug in the software which logs a user out immediately after login, in some circumstances. Refreshing the page may work for you.
If not, please use the contact form to describe the problem as best you can. Include your device type and browser. For instance, “Chrome on a Windows 8 PC “or “Safari on an iPad”.
More information on clearing your cache.
File exceeds allowed file size.
Try reducing the image size to @ 500 kilobytes. If the image is in png format, convert it to jpg as these are usually much smaller file sizes.
Often the best way to describe a problem is with a screenshot. An excellent tool for this is Greenshot, free from http://getgreenshot.org/