If you have a HubSpot account, go to Marketing > Files and Templates > Files. Upload the picture there and get its URL by clicking Details, then copy the URL and paste it into the generator and it should work.
Could you upload the image to Google Photos and then get the direct image address via a right click, like this:
Copy pasting this address into the email generator should do the trick.
If that won't work, I would recommned changing the format (jpeg or png) and the size of the logo. A good first step could be taking a screenshot of the image and trying to upload that.