This forum's software is about 10+ years out of date so it's trickier to add photos here than using an up-to-date forum.
First, you'll need a place to upload a photo. I recommend imgur.com. This is a site where you can upload photos for free and easily retrieve the URL for each photo. Create an account there and upload any photos you wish.
Once you've uploaded your photo, copy the URL address for it. It will look like something this:
https://i.imgur.com/sdoqRqj.jpg
Then, choose the area in the body of the post where you want to put the image. Paste the URL and highlight it. Then, click on the Img button in the editor controls above. That will place the proper html markups before and after the URL. In the body of the edit it will look like this:
{img}https://i.imgur.com/sdoqRqj.jpg{/img}
except that the curly brackets {} will actually be square brackets [].
Once you submit the post, the photo will appear such as this.
It's easy, if a bit old-fashioned, once you get the hang of it.