Click on “Choose File” near bottom left of the new window and browse your computer for the pic you want to use.
Click on “Upload” and the file will be uploaded.
Make sure it’s no bigger the 500k. Resize only makes it size down to fit the forum screen window and not shrink the file size. You can easily shrink pics down with paint but make sure you rename the file if you want to keep the larger (higher res) file as once you shrink it, you can’t blow it back up and keep the high res.