PNG's not showing in Firefox

I love this widget! However, I loaded some transparent PNG’s, and they are not showing in Firefox. They display and rotate perfectly in Chrome, but have weird black outline in IE. I have tried a couple different machines, and the results are consistent. I tossed in a JPG just to see what would happen, and the JPG displays on all browsers. Where there should be a PNG, it is just blank for the correct amount of time as if it were on display. What can I do to solve this problem?

See: http://kof-cms.fordandassociates.net/kof-cms/
(If you watch the blue header area long enough, the JPG will show. It’s one of five images, the other four are PNG.)

https://wordpress.org/plugins/dk-new-medias-image-rotator-widget/