Impress: animation lost after i change the image. Bug or not?

I have a presentation in Impress. All the objects in slides are PNG images. I have set all animations with them and released first version. After that I have compress all the source images. I wanted to change images in presentation to their “lighter” versions. But after i have changed first of them (using context menu “Change…”) it lost his animation.
The object have changed only one attribute, but it seems like it had been recreated completly. Think this is a bug!