The only time I would tag anyone other than the people actually in a picture are ones who would be interested. For example, when we went on a family camping trip I took lots of pictures and posted them when we got back. I tagged DH's aunt and uncle in the pictures of their grandchildren. Also, when my family back home gets together I will usually get tagged in at least a pic or two, especially if it includes my mother. My favorite one is one my cousin J took of her daddy(Mama's big brother), Mama, and their little sister L. They've already lost one sister and my uncle is suffering from Alzheimer's and quickly losing his battle. I love that they were able to get this picture and my cousin thought to tag me in it. Otherwise I might never have seen it.
I would never tag someone in a random meme picture, but what I have done is tag someone in a comment on it. I have several friends that I worked with at CellPhoneCompany and found a funny one about Nokia phones (those things will not die!), posted it and tagged my friends in a comment on it, something along the lines of "Hey xxx, yyy, and zzz, I saw this and thought of you guys." This is perfectly acceptable in our group. I wouldn't do that to most people though.