1. Take a screenshot with the Prt Scn button. Generally this key can be found above the Insert key and to the right of F12.

2. Open MSPaint or equivalent image editing software and paste the image in the canvas (Windows: Ctrl + V)

3. Save the image as a JPEG. Name the file after this convention: YYYY-MM-DD_##### (Year-Month-Day_ScreenshotNumber). If you take two screenshots today then they would be named 2013-06-28_00001 and 2013-06-28_00002. (Note: You can probably get away with naming them something else, but I would suggest sticking to Steam's convention)

4. Find your Steam folder and navigate to this path "..\Steam\userdata\70011386\760\remote\". On Windows Steam is typically found under C:\Program Files\

5. Now you'll need to find the appID number of the game you're playing. The best way to do this is to go to the store page of the game you're playing and look at the URL. The number at the end of the link is the appID. You can also find this on the game's community hub if it's not longer being sold on the store.

6. Create a folder in the directory you navigated to. This folder's name will be that of the appID. So if you're playing Unreal you would name the folder 13250.

7. Inside this folder, make a subfolder named "screenshots". Within the newly created screenshots folder make another sub folder named "thumbnails". The folder names shouldn't include the quotation marks.

8. Now place the screenshots you saved into the "screenshots" folder. Make a copy of every screenshot in this folder, navigate into the "thumbnails" folder and paste all the copies in there.

9. The next part is tricky, you need to shrink the copies down to the size of a thumbnail. The easiest way to figure out what size to make your images is to just make them roughly ~15% of the original images size. If you took the screenshot in fullscreen mode you can navigate to the thumbnail folder of another game that you've taken fullscreen screenshots in and check the dimension of the thumbnails. Once you've figured out a size, edit the images using MSPaint or your image editing software of choice and save the images.

9. Exit Steam completely and then reopen it. Go to your screenshot manager. If you've done everything correctly then the game in question should show up in the dropdown box. From there you can select and upload your screenshots to the Steam Cloud as you normally would for any other game.

Enjoy!