To convert an image file to another file type:

Open a file in Preview and choose File > Save As.

Choose a file type from the Format pop-up menu.
Depending on the file format you choose, additional options may appear. If you choose JPEG or JPEG 2000, you can adjust the image’s quality. If you choose PDF, you can choose a Quartz filter to apply to the image, and you can encrypt the image so that only those with the password can view it.

Type a new name or choose a new location for saving the converted file, and then click Save.