Wow, curious bug. And confirmed here! To reproduce (not always):
0) New browser
1) Goto http://ruralu.org
2) Press the login link
3) In the login page, press "login as guest"
4) What do you get?
In 4), sometimes I got redirected back to the site page (correct) but 3 times I've been redirected to images like http://ruralu.org/file.php/6/USGS.jpg
I only have been able to reproduce it randomly in FF (viewing 3 different images, all them present in the main page).
Perhaps we are redirecting to something incorrect (wantsurl, or refereer, or whatever messed by requests to images?)
Addressing to 1.9.1... curious, really curious... ciao