Steam will not show up in the game list if you use the Steam UI updated in 2023.
So far, your options are as follows:

• Revert back to the old UI (Launch Steam with the -vgui command option)
  
• Use an external tool from the Tools to automate the process section
  
• Use a different upload page and change the file_type to 5 (guide)

Assuming everybody knows how to upload screenshots from a regular game, this guide explains how to upload screenshots for "special games" like Steam.
You can for instance use this method to upload screenshots of games that aren't on Steam and thereby lower the amount of unrelated images to other game hubs. The result will look similar to this:

If you're looking for a quick and easy solution, you might want to take a look at these tools:

Steamscree: download[steascree.download]
Probably the more reliable and safer one. You can use it on pretty much any OS and it comes with some extra features such as auto-resizing huge screenshots, converting file types and several others.

Steam Screenshot Uploader: more information
An alternative tool that allows you to tag friends and add the location of the screenshot. Please note the high detection ratio on VirusTotal[www.virustotal.com]. I lean towards saying these are most likely false-positives, but it's impossible for me to know for sure.


In both cases you should use the AppID 760, which will show an empty game name.


I did not create or assist the creation of these programs and I made reasonable effort to ensure they are trustworthy. You will however have to decide for yourself whether to use them or not. Please also read the disclaimer section of the linked guide. You can simply follow the manual steps below instead if you do not want to use these programs.

Start by pressing Show on disk in the upload window.





Your explorer should pop up and there path will be similar to:

Now go down two instances, into the folder

You'll need to create three folders, the first one right here. The name has to be the game-ID, some examples are:

• 753 for the name Steam
  
• 765 for the name Greenlight
  
• 760 for an invisible name (actually is Steam Screenshots)
  
• You can find other ones either in this guide, or manually on SteamDB[steamdb.info]

Inside this new folder, you need to create the second one and name it screenshots. And finally in this new folder, you create the final folder called thumbnails.

Verify that you now have created the following path:

Simply copy & pasting all files into the folder does NOT work. You'll need to need to follow the following steps.

1. Copy all pictures into the folder called screenshots
   
2. Create a copy of every image in the same folder and name it [original name]_vr.jpg
   
3. Make sure you now have the two files e.g. image1.jpg and image1_vr.jpg
   
4. Restart Steam
   
5. Open the upload window again and navigate to your "new game". Wait a few seconds and your images should appear.

• Try to find the folder on your other hard drives (e.g. D: or G:)

• Make sure you are using the old Steam UI. You need to launch Steam with -vgui
  
• Do NOT play/idle games at the same time
  
• Try to restart Steam
  
• Make sure you have used the correct folder path

• Make sure to restart Steam

• Check the size and file type. You have to use .jpg files and make sure they aren't large files Maybe try to lower the size or the quality.
  
• Do not simply rename files (e.g. .png) to .jpg. You have to convert other file types to jpg. Open your picture in Paint and save them as a .jpg
  
• Make sure the "_vr copies" are named properly?
  
• Some games (which you either don't own, or do not support uploading user generated content; usually apps such as Steam Hardware) do not work. Try a different AppID. Experienced users can also try to check the AppID on SteamDB.

Feel free to ask other questions via a comment.

• The filename of the images has always been COMPLETELY irrelevant. You can name them whatever you want.
  
  
• You don't need to create a thumbnail. Steam does it automatically when you open up the upload screenshot window.
  
  
• You can also upload .png and .gif as a screenshot using a well known exploit in Steam https://steamproxy.net/sharedfiles/filedetails/?id=693118839
  
  
• Quite a while ago, Chudah found a great way to upload screenshots from non-steam games. Sadly the pictures doesn't show up in the feed anymore http://steamproxy.net/sharedfiles/filedetails/?id=518072312
  
  
• This weird, yet necessary procedure of creating copies is needed due to a Steam update in summer 2016. The original guide and other methods
  
  
• The here called USER_ID is actually your Steam3ID, which you can find using SteamRep[steamrep.com]

Since some people are concerned about this method and Valve's position on it:

I certainly believe that Valve and Steam's Volunteer Moderators are fine with this method.

If I wasn't, I'd certainly not share this information publicly. All of the following thoughts are speculations and assumptions based on previous events and statements.
Valve's main concern regarding user-generated-content seems to be to prevent users from scams/threats and keeping the game hubs clean from spam/unrelated content. I believe that is why Valve will continue to allow content which isn't linked to a specific game, like this guide, to be posted to these kind of "games". These games do not have a community hub.

This method might very well be comparable to the "exploit" of uploading guides to Steam:
Just by visiting this link you can create such a guide. Valve is undoubtedly aware of it (They have even used this method themselves), but they never removed this method, nor did they ever ban a creator or guide for using it. The reason could very likely be the one listed above; Valve is probably happy to see these guides not longer being posted to games like TF2 or CS:GO. They were disturbing and thus got flagged as Incompatible, or in extreme cases removed.
The moderator KillahInstinct on this topic.

Thanks to Legends Saga for encouraging me to create this guide, as well as SgtFopper and PsyKitty for their inspiring guides on similar topics.

The thumbnail was created by me, using an icon from Sweetclipart[sweetclipart.com], under the CC BY-NC-SA 3.0 License.