It doesn't clearly specify this in the Image Guidelines, but when taking a Screenshot try to remember to press Alt + Z to hide your UI. This hides your action bars, addons, the Target Circle, and other User Interface. This won't remove the name of the NPC though (which can either be cropped or zoomed out, or just hide names). The name is not generally preferred because it is either clearly stated on the article, NPC box, or even on the pictures uploaded name. On the Image Guidelines, it only requests no UI on location shots; but I think this would be greatly appreciated for NPCs too.
Not to fray too much though! I've gone through (still going through) all the images I can and replacing and shots with UI with a better version. Also, a tip to conclude my speech; the Wowhead team has become more strict with their Screenshot policy, if you haven't noticed. This is probably because of maybe a stricter team, or just rule changes. Whatever the case may be, they require you do use Alt + Z for your Screenshot to be accepted. So, if you're finding yourself in an odd situation trying to cut out all the UI, just head on over to Wowhead and borrow something from them.
Post a message on my talk page if you want more in-depth tips on taking nice clean Screenshots, as Wowhead has approved a number of mine.
Thanks for your understanding everyone!