First, you'll need a tool to take screenshots since the steam screenshot tool doesn't function properly in Waste Walkers. Many computers come with built-in tools like this. An example would be the snipping tool.

Once you take your screenshot, save it as a .jpg. In order to upload to steam, the screenshot must be a .jpg file.

Navigate to your steam folder on your hard drive.

Find userdata. Open it.

Find your steam id. Open it. It will be a numbered folder.

Find 760. Open it.

Find Remote. Open it.

Create a folder named "371100". This is the appid for Waste Walkers.

Create a folder inside the folder titled "371100" named "screenshots".

Create a folder inside the folder titled "screenshots" named "thumbnails".

Put your screenshot in the screenshots folder, then make a copy of it and put it in the thumbnails folder.

Restart steam.

Run the game for a second and then quit.

Go to your profile and attempt to upload the screenshots.

(Restart of steam may be necessary)

That should allow them to appear and you should be able to upload them.