You probably aren't inserting urls that are publicly available.  The forum software would have to have an api token to reach the url, which it does not.  I don't use dropbox, and don't post publicly from Google Photos, but look for a way to make your images available to non-logged in users.  Then you can either use the forum message editor to insert a link, or insert the image inline with the buttons on the toolbar

I was curious to see if it works, and I was not able to insert a shared album inline either.  Best I could do is a sharing an album and getting a public link:

https://photos.app.goo.gl/3vQcYaEBdHUXwrgYA

  Our best bet in the meantime may be what I mentioned above, which is to share an album on our platform of choice, and post a public link here (I unshared the album I linked to above, but it was reachable before I did). 

There's recently been a multi-part post with inline photos, perhaps that author discovered the steps.

Hi,

I'm the author of the recent multi-part post. The secret sauce is that you have to store your photos on a server that allows them to be accessed via a simple http link. The image can be in any format (jpeg, png, etc.) that web browsers can handle. One of my posts even has an animated gif showing a cockpit view of a Faering Cruiser under sail.

What kills the effort for most people is that they use an image hosting service that doesn't give them a simple, public and fixed link to the image file. The old image hosting services used to do that but many have gone bust, taking the images with them. A lot of the newer services add complications that no longer let you get the link to the image and paste it into the CLC image box. Google, for example, generates a new and temporary link to the image. If the poster remembers to specify that the image is public, then the link will work and the image will show up here, but after some time the URL expires and the image disappears.

I get around all this by having my own server. Years back I bought a domain name (they're reasonably priced), got it hosted and set up a repository for all the images that I want to share online. Then all I have to do is:

1. Take the picture

2. Post-process it to make it look good (cropping, rotating, brightness, contrast, etc.)

3. Upload it to my server

4. View it in a browser to make sure it's accessible

5. Get the link location for the picture

6. Paste it into the image box in the CLC post

That's it. Basically, I'm my own photo hosting service. If you can find a hosting service that doesn't get in your way in the name of security or making sure that viewers see lots of ads it'll work. A simple, public fixed URL to the image file is all you need.

Good luck,

Laszlo

I haven't used it in awhile, but imgur allows your hosted photos to be placed inline on third party sites.  I think the issue will still be this forum though; it has to support BBCode for inserting images, but I don't think it does.  I'll try later if somebody hasn't already.

   I found a way that seems to work by first copying my photo to Flikr, opening that image, right clicking then clicking "open image in a new tab" then right clicking the new image and selecting "copy image address".

Thanks to everyone that offered suggestions.