Working with KompoZer 0.8b3 on Windows 7.
If I open Link Properties, I have an opportunity to specify "Open link [in a new window, ...]". That is good.
If I open Image Properties, there is a Link tab to specify link properties (if the image is a link, like a button or thumbnail). This tab is missing the "Open link [in a new window, ...]" field.
WORKAROUND: Edit the HTML source directly.