Have you tried adjusting the layout padding? I have found that changing that to 0 not only makes the viz size larger it also does away with borders and empty space.
I'm not sure if this is what's going on or if this will help, but I'm getting the feeling that if the
size of the underlying graphic is larger than the image box, the white border appears when
fitting and centering.
Using a trimmed version of the icon as you suggested, I made a couple of tests.
The graphic size was 336 x 343
-I made a small image box (141x129) and the border appeared.
-I made the image box larger (337 x 349) than the underlying graphic and got no border.
267469icon2.twbx 1.4 MB
Thank you so much! I'm glad for an explanation of why the white border appears sometimes and not others. I wish they wouldn't add the white border if the image size is larger. It's a bit frustrating that I have to shrink the images before adding them in Tableau but at least it is an easy fix.
Just wanted to reply and say thanks for answering. That was my initial thought too but even with inner and outer padding as 0, the white border was still present. Swaroop.gantela had a fix (border appears based on image size vs. layout size) and it's working now.