Steve, I uploaded them to Imgur, https://imgur.com/ . Then, from the Imgur page for the image, I copy/pasted what is called the "BBCode" for the image, into my Forum post.
As you may know, Imgur is a free image-sharing site. You can create a free account and sign into it, but I think this also works if you upload an image without signing-in.
Once you upload your image (it's easy), you'll have its URL but what you want instead, to copy/paste into your Forum message, is what's called the BBCode. You get that by hovering over your image in Imgur after uploading, then in the little pop-up menu, look for the little down-arrow (V), to see several choices. Click on "Get share links," which gets you the "Share options" screen, where you'll see 4 choices including the BBCode. Click "Copy" next to the BBCode, then paste that into your Forum message.
You can also use Imgur to upload a custom avatar for your Forum posts. But, for your avatar, for some reason, you want to copy/paste the uploaded image's URL instead of its bbc code. (They're similar, but with some differences.)
I hope that helps -- it's pretty intuitive once you are in Imgur, except maybe for knowing to hover over your uploaded image to get to the "V" arrow for the Share options.
I used to use a similar, free service called Photobucket, which is still around if you want to check it out, but Imgur seems to be the most thriving platform for this now.
I recommend clicking the Preview button on your post, when you're adding images, to see if you've got it right -- but it's not difficult. The screenshot I uploaded was 720 pixels wide, but I'd say about 300-600 is enough. You don't want it too huge.