The general idea is reading the screen pixel by pixel, then having the client transmit that data to the server. You’d probably have to send it via datastream (easier) or setup your own system to send it through console commands. Once GMod 13 comes out you could easily use the net table to send it.
Then it’s up to you what to do with the pixel information on the server, you could upload it to a MySQL server that’ll in turn rewrite it to an image with a php script or something else.
(It’s very expensive to draw the image raw (pixel by pixel) in the server for admins to see, therefore I’m recommending that you have something that converts it to an image)
[editline]9th April 2012[/editline]
However you have to keep in mind that the table could be very large if you want to capture the entire screen, it might take a long time to transfer the pixels and the client could possibly even disconnect during that time.
The idea was to upload the jpeg to the server and display them out of the game, can be linked to a web server via a simple symlink relation to be viewed out of game or inside an HTML element. Not pixel by pixel.
Tobba was testing something similar on me a few months ago. Taking a client side screenshot using ‘jpeg’ and then uploading it to the servers data folder. Or screenshot folder, I forget.
I’ll ask Tobba how he did it when he’s back online.