You can resize or rotate an image opened in Preview to fit into a particular spot. You can also reduce the file size to save disk space or to download an image more quickly.
Crop an image
To crop an image, you first select the area you want to keep, and then discard the rest.
In the Preview app
on your Mac, click the Show Markup Toolbar button (if the Markup toolbar isn’t showing), then click the Rectangular Selection button .Select the portion of the page you want to keep.
If you want to view the dimensions of the content you’re selecting, choose Tools > Show Inspector, then click the Crop Inspector button
and choose a unit of measurement that’s displayed in the Crop Inspector window.Click the Crop button
in the Markup toolbar (or use the Touch Bar).Note: To undo the crop immediately, choose Edit > Undo Crop. To undo the crop later, find a version of the image before the crop. Choose File > Revert To > Browse All Versions, then browse the versions to restore the one you want.
(if the Markup toolbar isn’t showing), then click the Rectangular Selection button 
.
and choose a unit of measurement that’s displayed in the Crop Inspector window.Change an image’s dimensions
In the Preview app
on your Mac, click the Show Markup Toolbar button (if the Markup toolbar isn’t showing), then click the Adjust Size button .Enter new values for width and height, or click the “Fit into” pop-up menu and choose a size.
Resize the image by a percentage: Choose “percent” from the pop-up menu next to the Width and Height fields, then enter the percentage in those fields.
Force the image to keep its original proportions: Select “Scale proportionally.” If you change either the width or height, the other value changes to maintain the proportions. If you choose a size from the “Fit into” pop-up menu, the image’s width or height may be smaller than the chosen size to maintain the proportions.
Make the image’s dimensions smaller without losing any detail: Deselect “Resample image” before changing the width or height.
.Tip: To resize multiple images at the same time, display the images in the same window, select them in that window’s sidebar, then choose Tools > Adjust Size.
Reduce an image’s file size
In the Preview app
on your Mac, open the file you want to change.Choose Tools > Adjust Size, then select “Resample image.”
Enter a smaller value in the Resolution field.
The new size is shown at the bottom.
Tip: To reduce the file size of multiple images at the same time, display the images in the same window, select them in that window’s sidebar, then choose Tools>Adjust Size.
Rotate or flip an image
In the Preview app
on your Mac, open the file you want to change.Do any of the following:
Rotate an image: Select the image, then click
(or use the Touch Bar) to rotate the image to the left. Continue clicking to keep rotating.To rotate the image to the right, press and hold the Option key and click
until you’re done rotating the image.Flip an image: Choose Tools > Flip Horizontal or Flip Vertical.
(or use the 
until you’re done rotating the image.Tip: To rotate or flip multiple images at the same time, display the images in the same window, choose View>Thumbnails, select them in that window’s sidebar, then choose Tools> [Rotate or Flip].
