awesome work. I have some suggestions though:
1) The [Make Avatar]-link is available on the thumbnails page only. Why not add it to the individual image view (displayimage.php) as well?
2) You're of welcome to take credit for your plugin. However, I don't think that it's a good idea to display your email address in the link, as it is spiderable and will probably result in a huge increase of spam for your email address. I suggest adding a link to your home page instead (maybe the contact form).
3) The link "crop the image" at the bottom of the page is not clearly recognizable as a button in the first place. When I tested your plugin on your demo page, I nearly gave up after cropping the avatar, because I couldn't find the "apply"-button. I suggest using a button (using the CSS class "button") instead of spreading the final submit bitton over the entire width of the table.
4) The width and height attributes of the initial pop-up are too small, so you have to resize the pop-up window. I suggest increasing the initial width or (even better) dropping the idea to start a pop-up window in the first place and make the avatar creation page open in the original window.