Welcome to the forum. This is a task that I found embarassingly tricky myself, being a computer guru. The last time I was involved in forums, they were text only, tho they weren't called that. (Yup, an old dawg here.)
Basically you must first find a web server (site) to host the pictures, PhotoBucket.com in an example of one, there are many others.
Once you have your photo(s) uploaded to a (public) site, you copy the url (web address such as the one in your address bar, if it's displayed) to the clipboard. Then you post your message here where you want to share them. Then you add some text, like before the picture link, then add the URL prefix tool in the Post a Reply screen, then paste the url into the text, then immediately close the link with the /URL (close tag) tool.
Then you can post some more text, if desired. Of course the text before and after is completely optional, but usually a good idea. It can be short and sweet like: "Check out my latest rifle!" and then just add the link in betwen the tags.
So in review:
1 Load the pictures to a web site like PhotoBucket, Flickr, PhotoFish, etc. and grab the link (clipboard)
2 Begin your post to the Sigarms556 forum
3 Click the Tag button labled URL & paste your link in after it