How do I upload old screenshots taken with traditional methods to Steam?
Unfortunately, no. There is currently no official way to upload screenshots to Steam directly.
However, there are work-arounds on the Steam forums, with varying rates of success.
I personally would suggest waiting until Steam officially releases a method to do this.
Take a screenshot in a game with the Steam overlay. When you exit the game, a window will pop up with your recent screenshots for that game. Just click "View on Disk" and it'll open up the screenshots folder (folders are game-specific and named with numbers). Drop whatever pictures you want (Steam uses .jpg, not sure if others will work) in there, but be sure to make a 200x125 thumbnail of the picture in the thumbnails directory or it won't show up. You may have to restart Steam before they show up but you should then be able to upload them like normal.
Sometimes, I have run into issues, where it assigns a random time that the screenshot was taken, if that matters, and I don't yet know how to fix that. Also, you may have to use the same naming scheme as the normal screenshots (year-month-day_#####.jpg - 2011-09-10_00001.jpg, for example).
Take an in-game screenshot with Steam and then exit game. Open the Steam screenshot manager and choose Last Session > View on Disk. (Make sure you don't upload anything.)
Find the screenshot you would like to upload and put it in your Steam screenshots folder. (It should have opened from step 1.)
Copy the name of the screenshot you took with Steam.
Name the screenshot you took with Steam to something temporary.
Paste the copied name to replace/rename the picture you want to upload.
Close the Steam screenshot manager and then open it again.
Your picture will be available for upload but in the manager; it will look like the one you replaced. Simply select upload and view online and you will see the correct one.