As for the use in an advertisement. Since you were assembling this ad for a client, you were wise to take a prudent course -- particularly if the client included the term "terminator" -- in the ad copy.
So Many Terminators, So Little Time. The movie company that owns the rights to the TERMINATOR trademark (StudioCanal SA) has rights for the term in relation to motion pictures, computer games, amusement parks, paper products and games, toys and model kits. But they're not the only ones with dibs. Over thirty entities have registered TERMINATOR as a trademark for goods that include wheelchairs, metal grates, firearms, pre-moistened wipes, garbage disposals, silicon-based foam suppressants, fireworks, rock crushing equipment and those spikey-things you used to see in parking lots that deflate your tires if you go the wrong way. So you can see why, with all those uses out there it's best to keep Terminator references off of products or advertisements unless you're sure the use won't step on any toes.
Why so many "terminator arm" photos for sale? You asked why so many stock photos are available of the terminator arm? The Dear Rich Staff believes that photos of the "terminator arm" (such as the one above) have limited copyright protection. For example, you could create a similar model artificial arm, photo it and tag it "terminator arm." In other words, the bigger issue is not the image of the artificial arm (or similar artificial arm technology), but the use of the trademark 'TERMINATOR' in regard to a product or service, or the implication that the movie company (or one of the other trademark owners) is endorsing your product or service.