The common folder is in the steamapps folder, not the username folder
I'm sure the screenshots are probably somewhere on your computer, but are just misplaced wherever your pre-Steampipe installation of Garry's Mod is, because ever since the Steampipe update, some users had problems with Garry's Mod switching to Steampipe where it's folders would not move to the common folder and would instead stay in the username folder, or it WOULD go to the common folder, but save files in the username folder or some other broken nonsense like that