I know I've done this before, but I can't seem to do it now.

I've tried with
Dropbox
We transfer
Google drive
One drive

Either I am not understanding the syntax, or these are not the service I last used for such?

Thanks for any help

What do you mean by file embedding?  Is it so that when you pull the link up, it comes up in an audio player?

No...

So I can post visible pictures on forums and such.

It seems most only allow me to post links...

for images and animated gifs I use imgur.com
 

Yep, second imgur.

Keni
I've tried with
Dropbox
We transfer
Google drive
One drive

I use OneDrive without issue.  Select the photo, click on the Embed tab, choose your size and click Generate (leave "Include HTML tags" UN-ticked).  NOTE: the Embed tab does not appear when more than one image is selected - it may also be hidden if the window isn't maximized.  Click on the URL and copy.
 
In this forum, click on the insert photo icon at the top right and paste the code.  Hope this helps.  Your pic should appear immediately like this....
 

 
 

Quote this post to see the syntax