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Make Div-Block appear on mouse-hover: Size issue with underlying image

Hi everybody

I have the following situation:
4 columns filled with images, the images have different aspect ratios.
Each image is nested inside a div-block.

Now, for reach image, I want to use a div-block (with an applied background color), that appears on mouse-hover.
The problem: how can I tell this div-block, that it should have the same size as the (underlying) image?

Thank you in advance for any help!
Nicolas