Carnage said I should make a guide on how to upload any screenshot to Steam so.. Here I go
Step 1: Launch any Steam game (I prefer L4D)
Step 2: Take as many screenshots as you want to upload
Step 3: Alt-tab out (DON'T CLOSE THE GAME)
Step 4: Go to the place where your Steam is installed and find this folder "Steam\userdata\21916536\760\remote" and if you took your screenshots in L4D go to the "500" folder (440 is TF2 and 550 is L4D2) and then go to the screenshots folder
Step 5: Find the picture(s) you want to upload and copy them to that folder
Step 6: Rename the picture that you want to upload to the name of the picture that you took in-game, like "2011-03-25_00001" and when it asks you if you want overwrite click yes
Step 7 (Optional): If you want to make it look nice on your page you also need to make a thumbnail for it. Do what you did in step 6 but you also have to resize the picture to 200x112. I use BatchPhoto for resizing.
Step 8: Exit your game and upload the picture(s) you made and you're done!
Note: The picture will look normal on the screenshot manager but it will change when you upload the picture.
Leave a message here if you can't do it and I'll try to help you :)
I uploaded this picture while I made this guide xD