Downloads for Outlook email gifs etc.

Historically I have unchecked the down load from safe senders box in the trust center options. Worked fine, no downloads unless I say so. I totally wiped my safe sender's list and then checked the download gifs, pix etc. from safe senders list box in the trust center options. I did this to selectively recreate the list SAFELY. However I now download gifs etc. from each and every email, more specially, those NOT IN THE LIST. The only way I can stop it is to uncheck this box again. Then I, as I have in the past just selectively download individual emails. Of course, this stops me from getting hung by spam, or nasty emails etc. and is OK as a solution. But why do I download all when I check the "safe sender's download " box, as if they were in the list? The feature as I thought I understood it to work would be a much more elegant solution. What am I not doing or understanding here!! Thanks in advance for any help!!