Even on facebook with their picking software it does give you that option.
As an example,
someone shared their profile on facebook for people to add them there
It shows the default image as the only option. I think this is probably really bad if people keep seeing this same image on everything.
It would be much better if other platforms could detect an image in the post like when you share something from thedailymash or other blog sites, you get the image from their post. Im pretty sure this is an issue with vbulletin and not fb or reddit