Uploaded icon not centered in Features widget container

I have a single Features widget in a Page Builder row–the row has one column only–and in the widget I’ve set up 3 feature items. The container size is 150 px, icon size 75. I haven’t edited or added any CSS, HTML, or what not.

Here’s the issue: Icons chosen from the stock sets work fine, but an icon I uploaded refuses to center itself in the container. Instead, it shows up aligned hard left, hard top. Please advise as to how to center the icon.

The uploaded icon is a PNG. It has no padding–there are colored pixels up against all four sides. I’ve tried resizing the icon (started with a 96 square px image, 2nd try 75 px, 3rd 300 px), and fooling with the “Icon Image Size” setting, but nothing changes.

By the way, I couldn’t find anything in the Widgets documentation about how to prepare icons for this or other widgets (preferred format, preferred size, how to handle transparency). Maybe I missed it, but if not, please consider adding that stuff, wouldn’t take but a line or two. Thank you.