Join an internet photo hosting site. Like Photobucket. This is the one I use.
http://s917.photobucket.com/You upload your photos as Jpegs, (drag and drop into upload folder)
Click on photo and copy the link to share the photo.
Paste into the forum thread where you want the picture to be. You can press 'preview' to see if the picture is there as you want it before you press 'submit'.