When posting, under the box where you type it says "Attachments and other options", under "Attach:" click Choose File.
Then from there you locate the screenshot you took. For me it's:
Network > Your computer name (Mine is "Last name-HP") > Users > (Last name) > AppData > Roaming > .minecraft > screenshots
I don't know if it's the same for everyone, or if there's even a shorter way, but I hope this helps?