The image viewer in Mac OS is really powerful in terms of image manipulation. There are a lot of hidden featured apart from image and pdf viewing experiences. One of the least noticed featured in Preview is the batch image conversion.
For example, this feature can be used to convert a large number of JPG images to PNG images.
1. In Finder, select the images and open them in Preview.
2. Usually thumbnail area automatically opens up showing the thumbnails of the photos opened. If thumbnails are not visible, you can click on the top left icon to switch on “Thumbnails”.
Select the thumbnail by either Control+(select) or Command+A.
3. In the menu bar, click on File -> Export Selected Images …
4. That will show the film manager window in which, you might need to click on “Options” and then select “Format” to the desired destination format and finally click on “Choose” to export the files in the desired format.
For JPG files, export format are JPEG, JPEG-2000, OpenEXR, PDF, PNG and TIFF.