I found the issue. It is caused by the HoverImageUrl property in the RoundedCorners element.

I spoke too soon. When I click the button in Chrome/Safari, it would disappear, but the label still remains.

Nevermind, I found another way. Thank you very much for resolving this issue.

I used:

$(document).ready(function() {

Can I used css to accomplish this? I am using asp.net so I won't know the button's input element name.

Hi tnguyen13,

Thank you for your replies and for providing the used images. I was able to reproduce the matter and have logged this issue in our internal system with an id of 110184. Your support ticket number regarding this matter is CAS-77779-DGRFHL . Please feel free to continue sending us updates. You can view the status of the development issue connected to this case by going to the “My IG” tab on our website, hovering to the "My Support Activity" dropdown and selecting the "Development Issues" tab. 

The issue seems to be caused by the image button's input element being rendered with width of 1px under Chrome (which contrasting with the dark-colored images used appears as a dot). As a temporary solution I can suggest manually setting the width of the element to 0 on document.ready(). For instance:

        $(document).ready(function () {
            //this sets the width of the image button's input element to 0px
            $('#WebImageButton2').width("0px");
        });

Please feel free to contact me if you have any questions.