Size of inserted pictures in Visio is incorrect

[Inge]

Each time I insert a picture into a Visio drawing (Insert -> Picture -> From
File...) the size of the picture is incorrect. In Visio, usually the size is
larger than the original picture.

This happens with the image types PNG, GIF etc.
It also happens with Copy+Paste from the clipboard.

Do you know the reason? Maybe Visio can't calculate the size because
pictures are sized with pixels and Visio doesn't know pixels?

Is there a workaround?