We had the same issue, and after a bunch of testing today I figured out what causes it.
If you are using a custom scheme, the latest version of Wimba Create requires that the scheme folder in [C:\Program Files\WimbaCreate\resources\en.lproj\schemes] cannot have any spaces in it. Example:
[C:\Program Files\WimbaCreate\resources\en.lproj\schemes\My Custom Scheme] <- embedded images will not work.
[C:\Program Files\WimbaCreate\resources\en.lproj\schemes\MyCustomScheme] <- images will work.
Hope that helps.