Grant access to CCK referenced nodes
You could dynamically add the the same tag to the referenced image nodes when the 'object' node is created (or updated). To do this, you'd implement hook_nodeapi() and check the $op parameter for either "insert" or "edit" (depending on when you want this operation to happen). Then you can pull out the node id's of the 'image' nodes being referenced and apply the terms. This way they have the same taxonomy based permissions at the 'object' node.

There may be other ways, but this seems like it should work for you. I actually do the same thing to set a CCK node reference value during creation. I don't have access to a coding environment right now, or I'd add code snippets.