I thought the tag "tagme" was supposed to be used when uploading pics of ourselves, yet i see users here using on uploads that is clearly not them.

Like the user freakyguro that uses it on almost all of their uploads which is of all kinds of people.

I always assumed its purpose was to invite other people to add tags to the post... But I don't know for sure...

It is for other to add tag(s) that they thing are needed

If you want to tag your own pics I think is better to add the name of your account rather than just "me"