I don't think there are any official sources of high-res wallpapers or merch available, although that may come later once the game is finished. For now I can only suggest that you use F12 to capture your own in-game screenshots. They will be stored in your STALKERSOUP\_appdata_\screenshots\ folder. There is an option in the game launcher to save high-resolution TGA images - see under Game Manager - Launch Options. (NB: I have never tried this, so I don't know how good it is...)
If you want to use a specific design such as the FSS logo, I suggest that you PM TecnoBacon direct and ask him. (Click on the banner picture above, then on any TecnoBacon link.)
I can't speak for the developers, but personally I wouldn't see any problem if you used your own in-game screenshots for wallpaper or t-shirts designs, as long as they remain non-commercial and for your personal use only. For anything beyond that, you would need to seek permission from the copyright holder(s).
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Calendar
Subject: Wallpapers and logos 
. I want to customize my PC and make a t-shirt for me (non commercial of course) alternatively if there are some merch please share the links. 
