Perhaps the VI is in memory but the front panel isn't. The error might not make a distinction between the two situations. In any case, something to try is have the VI you want to get the image from open but minimized and see if you still get the error.
Also you can force LV to keep a VI's front panel in memory by having property node in the diagram that are linked to a control on the FP--which is something else to try.
Mike...
PS: if this doesn't work, can you post a small bit of code that demonstrates the problem?