It's a bit fiddly to start with, but not that difficult once you've done a couple.
But one thing to bear in mind is that the forum won't compact photos which are too big, so wide photos won't be shown in full. So maybe 600-800 pixels wide is about the maximum size in my opinion, 600 is neat and tidy, mabye 800 if you want to show more detail.
But, basically, you can show any online photo on here. Just right click on the photo in your browser and on your menu you should get an option something like 'Copy image location' (which is in Firefox - other browsers maybe slightly different. In Chrome it's 'Copy image address').
That'll copy the link to the photo in your memory. So just paste it in here (I use shortcut Ctrl-V for pasting) and you'll get something like:
https://i2-prod.stokesentinel.co.uk/inc ... G-0817.jpgPut the Image tags at either end of the link, which means it will show as a photo on here, so:
<img>link here</img>
You can just type them on, or highlight the link and use the 'Img' option above the message window to add them for you.
That's fine for photos on other websites etc, but if it's on your hard drive then you need to get the photo online to start with.
So you need some sort of photo host website. There are plenty around, but if you don't use one already, I use Imgur, because you don't even need an account.
So go here, and it's worth bookmarking if you want to do photos regularly:
https://imgur.com/uploadThere's a few ways to do it, but if it's on your hard drive then click on 'Choose Photo/Video' and that should bring up your hard drive in the usual way, then select the photo you want to show.
That should upload the photo to Imgur, and voila, that's it ready to go.
So just right click on the photo as described earlier.
Here's a random photo of the Nicolacopter

One thing to watch out for is that once the photo is uploaded to Imgur, if you put your mouse over it it'll show a 'Copy Link' option. DON'T use that, because that's a link to the whole page, not just the photo. Right click on the photo, and copy the link that way.
Another thing to remember is that it's best to get your photo to the right size (800 width max, as I said earlier) before uploading it to Imgur. You can resize photos using Imgur itself, but the one time I tried it I couldn't get it to work, so maybe easier to resize it before uploading it.