I don't have IE7 to be able to test this. It seems to be working fine in other browsers. I don't see anything obviously wrong in your source code. And I can't find any other reports of this in searching this forum.
Try to reproduce the problem on a default style. Create a new style with no parent and change only the title image:
Admin CP -> Appearance -> Styles -> Create New Style
Then select that style on your forum. If the problem goes away when using the new style then you know it's a problem with your custom style.