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I wouldn't use Aviary it seems to be unreliable at this time from a number of reports here.
Try another facility like pixlr.com or a download like gimp. Both these are free facilities and work reliably.
Ideally your image should be adjusted to the size you wish to display and the file size should be around 20 - 50 kb for a postcard size image and approx 100 kb or so for a full screen image as a jpg. Larger if you are using a png file format.