Hi and welcome to the over-complicated world of online photo sharing
Firstly, you'll need somewhere on the interweb to host the pictures - if you don't have your own personal webspace, try something like
Flickr or
Photobucket. Set up an account, and read up on their limitations etc, and most importantly, upload some photos. Not sure about flickr, but Photobucket allows you to resize the photos on or after upload - for this forum and many others a general guide is about 640 on the long side of the picture to avoid stretching the forum layout horizontally or not fitting the whole image on one screen vertically.
Once you've done that, you need to make a post as usual and just point at the image you want to post using IMG tags in the same way you would a URL tag - photobucket makes it easy by giving you [ img ] code which you just copy and paste into here to display your pic.

:
For example, this code links to a picture of homer simpson...
...and then you can see it in 'preview post' to check you've got the correct pic/looks ok before submitting:
Hope that helps!!!!
EDIT: Footnote - the code example above is an image, otherwise you wouldn't be able to see the physical source code... just thought i'd point that out in case anyone asked how it was done 