Adding a logo to a dashboard fails?

Perhaps the webserver hosting the image blocks hotlinking or blocking with CORS? Try opening web-developer tools in your browser (F12) and checking the console tab while attempting to load the image in the widget. It may provide hints on why it's not loading. Try using different images from other places (like the google home page) to see if any image works.

Mike_Moniz is right that this is probably CORS.

There is an alternative approach - include the logo itself in the markup (works if the logo is small).  Here's how to do that...

In an ideal world, the logo is SVG, and you can just include the logo in the markup:

If your logo is an image, you can convert it to Base64-encoded and include that with a regular <img src=>:

[Remember to ignore the fact that LogicMonitor STILL have an insecure version of CKEditor in use - facepalm.]