Adam,
I have replied to your question in Builder support forum here.
I am pasting it below for the benefit of everyone else who might come across this in the future.
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The reason for the button images backgrounds not appearing in IE is because the gradient backgrounds are set in wp-content/sp-resources/forum-themes/default/styles/default.php?color=cool-blue to appear only in Mozilla specific browsers like Firefox.
Example:
#spMainContainer a.spButton,
#spMainContainer .spButtonAsLabel {
width: auto;
height: 21px;
text-align: center;
line-height: 1.6em;
padding: 1px 7px;
margin: 2px 2px;
font-size: 80%;
font-family: inherit;
outline-style: none;
color: #555555;
text-decoration: none;
cursor: pointer;
background: -moz-linear-gradient(100% 100% 90deg, #ffffff, #d0d9e3); background: -webkit-gradient(linear, 0% 0%, 0% 100%, from(#d0d9e3), to(#ffffff)); background-color: #ffffff;;
border: 1px solid #bbbbbb;
-webkit-gradientborder-radius: 5px; border-radius: 5px;}
Notice the Mozilla vendor specific CSS property,
background: -moz-linear-gradient(100% 100% 90deg, #ffffff, #d0d9e3); background: -webkit-gradient(linear, 0% 0%, 0% 100%, from(#d0d9e3), to(#ffffff)); background-color: #ffffff;;
in the above.
For gradients to appear in the backgrounds for all browsers, you will have to either add CSS code for other browsers or use images.
http://www.colorzilla.com/gradient-editor/
More @ https://www.google.com/search?q=css+gradients+in+ie