Use the Tools > Reports > Unused File menu item. Any file that appears in the report can be deleted in Windows Explorer.
While I agree with Colum that this report should be used, I'd be very hesitant to just take the report at its word and wholesale delete items.
What I might suggest is to create a "purgatory" folder. Move the files to that folder until you are certain none of them are possibly needed. Then delete them after a certain time period.
Willam's script is very handy. You can also use it for other file types you don't need by changing the extension it searches for.