You have been bitten by the brittleness of GUI Scripting solutions – viz. their dependency on the UI element hierarchy of the target application. In the case of Mail.app, this hierarchy has changed rather drastically with Lion (no surprise there, as Lion’s Mail.app was a rather drastic update from earlier versions). The correct UI address of the “Load Images” button is now
button 1 of UI element 1 of row 1 of table 1 of scroll area 1 of window
As this is rather a mouthful to chew on, I’d recommend modifying the original script to operate on a reference, changing the core GUI Scripting command as follows:
set loadButton to a reference to button 1 of UI element 1 of row 1 of table 1 of scroll area 1 of front window
if loadButton exists then click loadButton
Note, however, that this does not actually reproduce Gmail’s behavior – it will just open all mails meeting the rule condition(s) in a viewer window with inline server images displaying each time the rule is run. It does not affect inline message display (in the message preview column), nor does it apply when you open the message in a new window yourself. Barring writing your own Mail.app plugin, there is nothing you can do about that, as the only event processing mechanism Mail.app offers you can hook into with AppleScript is executing rules.