Hi J. First, if you go to forum help and do a search, there's been some well written toots over the years.
In brief- you need a .jpg file, not a .pdf.
You need to upload it onto a website so that it gets it's own url address. Most folks here seem to use "photobucket"- it's free.
Go there, join and follow directions. They've made it pretty simple.
When you want to post a photo here, actually what you are doing is setting up a link.
Have your photo account site open at the same time as here.
In this reply window, click "use rich formatting". Up top there is a little icon that looks like a film strip. To the left of it is something resembling a monitor I guess.
EDIT: well that's annoying- half of my post disappeared. HRMPH!