If you hit ctrl-F11 in Divinity, the game will save a screenshot in the the '..\Divine Divinity\Run\capture' folder, as a png file if you have the patch installed, jpg otherwise. This does not work during cut scenes or conversations, but external screenshot programs can be used then.

Once you have the screenshot, you may want to crop it or add text descriptions, then save it as a (most often) a jpeg. Png files are higher quality, but usually significantly larger.
graphics programs, for editing screen shots

Next you need a web host (your ISP may offer you some space) where you can upload the file, to be linked to in a forum post. The following topic has some info, though you would not need hotlinking (you could link to a html page with the screenshot, rather than the screenshot itself).
free web host with hotlinking? (for avatars)