Gestalt : Figure - Ground
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We tend to put more emphasis on the figure in an image rather than the ground. The user will view the figure as more important than the background.
When both the figure and the ground have equal weight as in the example we either view the silhouetts or the vase as the figure we cannot view them both at the same time.
Escher is well know for using ambigious figure / ground relationships: http://www.mcescher.com.
It is going to be rare if ever that you will have a need to create an ambigious design for the web. Normally there needs to be a clear cut distinction between the figure and the ground.