Where does steam on PC store screenshots

If you mean game screenshots, they are stored usually within the game itself. Different games have different folders, the commonly-known one is Garrysmod's folder which is in C:\Program Files\Steam\steamapps\<Username>\garrysmod\garrysmod\screenshots. Other source games like HL2 may store it in an area similar to the above except <Username>\half life 2\hl2\ is where it is stored (no screenshots\ folder).