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adatapost's answer is right.

You can attach the events of all the picture boxes in one go.

After coding your event handler for one of the picture boxes select all the picture boxes.
On the properties window select the events button at the top and scroll to find the MouseEnter event.
Click on the drop-down arrow and select the newly code MouseEnter event handler.
Now all the picture boxes are attached to the event handler in the forms InitializeComponent() function.