URL as query segment

[eluser]d910qf[/eluser]
I should probably explain! The reason why the url (whether encoded or not) needs to be there is that the idea is that people can embed thumbnails in this way:

<img src="http://myapp.com/thumbnail/urlgoeshere/" />

I may add extra parameters for height, width, format etc. I can't believe this is so tricky to accomplish - is there any way to disable CodeIgniters automatic urldecoding on a per controller or function basis?