In your admin cp go to Plugins and Products -> Plugin Manager then scroll down the bottom and click add new plugin.
In the box for "Hook Location" Scroll down to you find "global_start"
In the title box put in "Max Images"
And paste this in the "Plugin PHP Code" box:
were it says add as many usergroups as you want
if (is_member_of($vbulletin->userinfo, 6,5))
example of my one:
and change the max images to however many images you want the max to be for those usergroups.
make sure the plugin is active when you click save.