You'll have to have the pic already on the web someplace (a picture hosting site) and you'll put a link to that pic in your post by clicking the Img tab when you're creating a post. This site will then know to access and display the pic.
Photobucket is the web hosting site I use but there are many others. You create an account there and then upload your pictures.
You probably want to resize your pics smaller for display to something like 1200x800 before you upload them. I use a free program (Irfanview) to edit and resize pics.
Profile pics are usually even smaller. There
should be a max size listed where you change your profile.
Try searching this site for "posting pics" or something like that.