You have no idea how much time it took to take that perfect selfie. You have no clue how much was paid to the awesome photographer that took that picture. You don’t know how many times that caption on Instagram was edited to fit that picture. There will have been no hate if you took the picture and just reposted it, but to watermark it… totally uncool.
It is wrong to take people’s pictures and watermark them as yours. The only excetion that there is in this case is if you edited the picture and added something new. If that is not the case, then you have no business watermarking it.