/sites/default/files images not showing up
I appreciate that this is an old post but after days of trying to resolve the problem I have seen many of these posts and want to update with my solution.

My web host is classed as a shared web host which means for me that I don't have as much access to the server side as with other hosts. The control area for the admin on my host is custom and lots of stuff that I have with another host and previous hosts id missing.

I resolved the images not showing by adjusting the .htaccess file. There are 2 that I have found so far in Drupal 7.24 (one in the site root and one in sites/default/files/) and I had to adjust both of them by commenting out the options section and the symlinks section. There is info on this by googling "one.com Drupal Installation".

Once both htaccess files had been edited and the permissions had been changed on the "files" folder in "sites/default/" to "755" the extra folders where created when I uploaded my images and the images started to show fine. If like me you are unable to change permissions within your website hosts control panel you will need to do it with either an ftp client or via ssh.