Not sure which version of Tableau Server you're using but we did have a known issue in the 9.2.x series where Custom Shapes appeared normal in Desktop but distorted in Tableau Server.
This should now be fixed in 9.2.6:
Custom shape images appeared stretched and blurred when rendered in a browser.
I am still experiencing this issue at 9.2.6. Do you know if it has been reported?
Here is what my example Workbook looks like:
Here is what it looks like on the Tableau Server Menu (looks great):
Here is what it looks like on the server:
Any Ideas? Thanks Mark or anyone else out there that may have the answer.
Sorry for the delayed response. It's possible you may have run into an edge case not addressed by 9.2.6.
More than likely it will be best to open a support case but before doing that, it might be worth trying the following:
(a) Is the behavior the same regardless of browser? Only because we've seen in the past where an image may not render correctly under one browser but looks fine in others
(b) Typically client side rendering is turned on by default meaning the client browser is used to render the image. But you can add a URL parameter at the end your URL to have it completely rendered by Tableau Server. So at the end of your URL it would read ?:render=false. Would be interesting to see if there's any difference in behavior.