Popup layout is screwed up in IE (works in everything else, even Edge)

PopUp Pro is installed with minimal configuration. We are using a custom size (780 x 300) and with minimal CSS customizations (pasted below). However on IE (not Edge!) the sidebar image is blown up across the top which screws up the entire popup. The image should be flush left. Is there a trick to get this to work or is this just IE being IE?

The custom CSS is attached:

.popup-buttons {
margin-top: 32px;

.wdpu-head {
text-align: center;

.wdpu-text {
background-color: #f0eeea !important;

margin-top: 60px;
font-size: 150%;

.wdpu-msg {
background-color: #7b1616 !important;

.popup-buttons a,
.popup-buttons a:link,
.popup-buttons a:visited
font-size: 1.4em;
font-weight: bold;
background-color: #7b1616;
color: #f3d173;
display: inline-block;
padding: 10px 20px;
margin: 5px;
max-width: 50px;
text-decoration: none;
border-radius: 4px;

#popup H1 {
line-height: 1.2em;