I'm getting a little tired of the same old GMail themes from "random". GMail lets you use your own background image by giving it a url. Is is possible to give it a url that changes every once in a while, giving a much wider range of themes? If the answer is yes, what is a source of such a thing. I was thinking something like Flickr would be a good source, but I couldn't find anywhere that had an image url that changed on a regular basis.
This might do what you want. It calls various-sized images from Wikimedia sites using a stable URL. They might not be the prettiest sometimes, but could be worth a try.
Following on from Andrew Lott's answer, I've found the following URL works really well:
Note that changing the dimensions at the end of the URL resizes the image - so the above returns a picture 1600px wide by 1200px high. Set it to your monitor's pixel width & height for best results.
In Gmail's settings under 'Themes', you can pick the theme 'Random', which will apply a random theme daily.
See this blog post on Google System from 2009.
I don't think it's possibly to shuffle only the background image.