Could you try resizing the viz in tooltip sheet to be 'fit entire view'? This can be found as a dropdown at the top toolbar.
Let me know if this works!
Thanks for your reply. Since it's a web page object placed onto the dashboard I'm afraid that won't work, right? At least, I'm not getting the fit entire view option.
Hey Bodie! This is actually pretty frustrating and there seems to be no fix, other than resizing your images externally before the URL call.
You can vote up the Idea present in the Community since October 2014: https://community.tableau.com/ideas/3838