I have logins setup for each member of my family. If I download a new wallpaper that my kids might like to use, I need to put it in /usr/share/wallpapers for them to have access to it.
However, I do not have write permission to that directory, instead I have to change to su, copy the files and change back.
Would it be unwise to change the permission of that directory so that I can write to it, or would I be defeating the purpose of the security?

If you are solely changing the permissions on the /usr/share/wallpapers directory I don't see why it's going to be a problem at all. I wouldn't go changing permissions on the whole /usr tree but just that subdirectory you made shouldn't hurt anything.

Another way (if you were just going to change world permissions on the wallpapers directory) is to add a group (let's say "wallpap"). You can then make your account a member of the "wallpap" group. Then, by chgrp'ing the /usr/share/wallpapers directory to "wallpap" then you can change the group permissions (not world permissions). This is a little nicer since you aren't giving everybody write access to that directory.

It's a tad late for me...sorry if what I said isn't clear.