Upload it to where? If you mean you upload it to the Trainz Gallery then it should show in the Gallery - it won't show in your forum posts. Or are you saying it doesn't show in the Gallery?
There just sign in using your username and password, same U use for the forum, and then click search, plug in your forum name, and U should see your screen shot ---
Open Windows File Explorer.
Go to where your T:ANE data is - usually C:\Users\(your username)\N3V Games\TANE\Build xxxxxxx
xxxxxxx = some sequence of letters and number.
Open the screen shots folder.
Look for your picture.
Go to a website that hosts pictures such as www.imgur.com
Upload your image - you can drag and drop if you want.
Click on the image.
Click on the download arrow on the image.
Click on sharing options
Click on the bbc-code forums to highlight the text.
Click on copy.