Yup, in fact, you could get really creative and do:
host/3qhOS3/3511.jpg
Now it looks like 3511.jpg is in its own directory... To make this work, you'd need to setup .htaccess in the same directory as your script and do:
RewriteEngine on
RewriteRule ^([0-9]+)/([0-9]+)(/?)$ yourscript.php?u=$1&i=$2
Notice the ([0-9]+)? They're seperated by a / and that / is what makes 3qhOS3/3511.jpg work. But you don't want to use just 0-9. You'll want letters in there as well. Untested, but I think you'd need ([A-Za-z0-9]+) but don't quote me. Check out the .htaccess regular expression docs to learn how to capture the URL input.