No, you can't.
All you can do is post image URLs (jpegs etc) using the icon showing the picture of the mountain. The actual images are stored on the URL server.
General URLs (e.g. of an AENA PDFs) can be included using the icon showing the chain link with the round blob above it. I see you have sussed this already.
Most forums do not allow the forum server itself to store binaries, due to the size. These (files, images, etc) can be referenced by URL but need to be stored elsewhere. Pics can be stored on Photobucket, for example. For files there are various 'free' repositories on the internet, but I don't think any of them keep the stuff for long.