It's not a forum issue (yes, it's old and leaks oil and needs to be kicked started once in a while) PNG files work fine.. See below
It's a Google issue that is spitting out links that aren't compatible with anything but Google
There probably is a work around, but Google will just change the line in the sand and you are back to square one.. (remember Google maps once working in the forum)
The simplest fix is, if you want to post pics here, they must end in.. Jpg or .png and be visible while you are not logged in to google account
Easy way to check is cut and paste the link that ends in .jpg/.png into a browser that you don't usually use (iexplorer) and if it works there, then theoretically it should work in here..
Sorry, the fault lies with the google links, not the forum