There are some spiders or something on top of the mountain near the imp castle in the Dark Forest that you can not reach normally (ranged attacks or Flash Attack will kill them). Other NPCs in unreachable places are most likely due to a bug/glitch with NPC placement or movement, or possibly were in areas that changed before the game was released, and got stranded.
Your ISP may also offer web space, and should allow hotlinking (linking directly to a file from another site). If they have multiple email addresses per account, you could set up one address / web space just for games.
Using a web host without hotlinking, you would need to provide a link to a web page that has the screenshot (which would be better for larger images anyway), rather than have it display directly in the topic.
When a forum topic is displayed, the text, avatars and any screenshots can all come off different servers, so Larian does not need to store avatars/screenshots anywhere.
