It's easy.
Go here 1st.:
http://www.imageshack.us
Then, click browse.
Next, open your RCT2 screenshot file
Click the SS you wand and click CHOSE
Now click upload
When you get to the next page, highlight the URL that says sutible for sites and forums.
Then hit EDIT COPY
Now go back to our post and hit EDIT PASTE
when you click submit, it will be a pic.
Dont do what i did, make sure BBcode is on.