At the bottom of your message, there is a tab just below your text that says upload a file.
You can click on it and it will access your file location on your computer, probably in your photos. They will have to be below the maximum size the forum allows.
Another option is to use a photo hosting website and upload the photos there, and then to use the "add image" button, which is located above your text in the extra tabs (it's the one to the right of the little smiley face that looks like a picture of a mountain with a sun above it)
I also moved your topic from the Mossberg announcement forum to the site support forum.