A friend told me: 'On the site area 51, people will put their own proposals up.' I looked up on the dictionary and the best match seems to be 'place something in a high or upright position'. Does it mean that it's an image: you put up your proposals like you would put up a poster on the wall, or is it another meaning ?
"On the site area 51, people will put their own proposals up."
It carries the same meaning as saying that people will publish their proposals on the site, for others to see and review.
Does it mean that it's an image: you put up your proposals like you would put up a poster on the wall
It's supposed to evoke that. But the idea of using "posters on the wall" in a public place as a way of giving ideas for review isn't really how things worked prior to the Internet. You would more likely have to "submit your proposal to a committee" (or something like that).
Physically speaking, prior to the Internet there were objects called "Bulletin Boards"...which still exist somewhere, I think. :-)
The term was later adopted in the computer world as a metaphor for online places where people had chats, forums, cat pictures, etc:
So the metaphor of posting (e.g. sticking a sign on a post in public) or putting something up has been embraced by the Internet to mean "public discussion". Putting things "up" might also have some momentum due to synergy with the term "upload".
I imagine this is a pretty "modern" way to make such a statement, that wouldn't have been in use 20 years ago in this way (nor would "site"). But note there is another similar phrase which is to "put something out there":
Person1: "The coffee here at Denny's is terrible." Person2: "Well...instead of coming to Denny's every Saturday we *could* go somewhere else. I'm just putting that out there..."
I don't know exactly how long that way of saying you are "making a suggestion" has been around.
(Note: Another meaning to be aware of is that to say one "puts up with" something, that means that you tolerate it, e.g. "I can't believe he puts up with her annoying cat.")