Yes, at first glance it would seem the easiest way to do this is to simply try to deserialize some of these objects using Java 6 and see if it works. If the serialVersionUid is wrong, you will get a clear error message. If there are a lot of these objects and you can't
test them all, then at least try testing as many of them as you can, using a random sample.
What sort of Image is this, really? I assume it's not a java.awt.Image, since those are not serializable. In general I'm suspicious of the idea of using serialization for image data - I would recommend using java.awt.ImageIO and its read() and write() methods to do this. But it sounds like you may be stuck with existing data that was written a certain way, OK. If you are using Java serialization, what exact class or classes are the serialized objects from? And, is it a class that has changed implementation at all in JDK 6? Is it a class under your control, or a library class of some sort?