Mouse Down Blow Up on Icons.
Simply hold down your mouse on the icon of an
image file and the image will instantly pop up in original size until you
release the mouse button. There's never been a faster way to check the
contents of a photo, web graphic, animated gif, PDF, or PSD file.
Mouse down on the icon pops an image. (Animation)
Yes, you can even pop animated GIFs.
This option has to be ticked: Configuration | Mouse Down Blow Up | On file icons as well.