Nov71;
The advice to upload your image(s) to a hosting site is good - Image Shack is fine and there are others.
Once the images are uploaded to the hosting site you've chosen, they are assigned a "URL" or a link. The link to your photo on the hosting site is pasted in forums such as this one.
Copy the link, (highlight it on the hosting site, then click CTRL C). Navigate to the PPRuNe page/thread you want, click on "reply" to open the thread (I know all this is blindingly obvious but for clarity I'm not skipping a step). The click on the small yellow image of mountains, which will say "Insert Image" - when the small dialogue box pops up it will have the "http://" portion of the link ready - you can either delete that portion and click on CTRL V to paste the entire link to your hosted image in the space or you can omit the "http" blahblah part - at this point you can experiment!
You might shrink your images before you post them so they aren't twice screen size, for example - try 640 x 480 for message boards.
The key is to use the correct link as different forums require different links. PPRuNe seems to require the "Direct" link, (the one beginning with "http://" etc). That worked for me when I was trying to upload a while back. Good luck.