Zoom tool
The zoom allows you to see small details on a picture.
It's a best tool created in the computers.
And the PictureViewer allows zoom, the biggest zoom set is 10x and the smallest zoom is 0.1x.
The ways for this tool
First: Menu bar
Go to Zoom > Zoom in to increase the zoom.
Go to Zoom > Zoom out to decrease the zoom.

Second: Icons below the image
Click in magnifying glass below the image to increase or decrease the zoom.
The magnifying glass with a minus (-) sign decreases the zoom. The magnifying glass with a plus (+) sign increases the zoom.
Third: Keyboard
Type - (in the numpad part of keyboard) to decrease zoom. Type + (too in the numpad part of keyboard) to increase zoom.