Screenshots
So I've taken a screenshot and uploaded it to the workshop with my level. How do I make it the screenshot that appears when you search the level (in other words the opening screenshot?)
I strongly believe that the camera position in your PTI map is the only screenshot that will be displayed. I think it?s not possible to change it. 
You need to upload your level from p2map_publish. There you can choose a custom screenshot. p2map_publish is in your Steam/steamapps/common/portal 2/bin folder. You'll find the workshop maps in steam/steamapps/common/portal 2/portal2_dlc2/maps/puzzlemaker.
I believe this is right, correct me if i'm wrong
[Released] Power Interference
[Released] Rust and Dust (Spotlight Map!)
[Released] Multiportal
UsCobra is right: if you want a custom screenshot use the publishing tool.
But personally I think it's a great system. You can easily pick out the PTI maps and generally speaking they are not as good as hammer maps. (there are a few exceptions, but generally speaking hammer maps are better)
Damn. Thanks anyway...
You can change or add images via the map's workshop web page. (On the right side, select "Add / edit images and videos".) It should also be possible to set up a slide show of multiple images.
(There is a "view in workshop" button somewhere in the in-game interface that takes you to this page, but I don't remember where exactly...)
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
@HMW - that's true, but I don't think you can change which screenshot shows up as the map's thumb in searches from the Workshop page. As far as I know, the p2map_publish tool is the only way you can do that.
My maps: Double Bill / Jam / Oval Window / Gymnasium / Gymnasium Part 2 / Minimalism (collection) / Resolution
All feedback welcome!
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Oh I see, you're right. It says you can upload additional images, besides the first one.
Welp, p2map_publish it is then.
(That being the case, it's extra annoying that the PTI-supplied image is just barely too small for the space it gets on the web page, resulting in that ugly 'banding' effect...)
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
Hi, I'm new here and I'm having trouble doing this myself. But what's strange is that I somehow did it before twice, but now I can't do it on my laptop for some reason.
http://steamcommunity.com/id/Unowninato ... ?appid=620
The one I want to change is Strange Cube.
Edit: Oh, and I went to SteamAppscommonPortal 2binp2map_publishresource, as someone else said, and it's just an empty folder.