Open the desired images in Preview. Select all of the images, and choose File > Export Selected Images. Then, choose the new location for the images and click Options. Pick the desired format and click Choose.

1) In Finder, command+click each of the images that you would like to convert. This will select all of them. Then, double-click one of the files. This will open them all in the Preview app.

2) In your Mac's menu bar, click Edit. Then, click Select All.

3) Click File in the menu bar. Then, click Export Selected Images.

4) Choose the location where you want to store your converted images. Then, click Options.

5) Click the "Format" drop-down menu and pick your desired file format. Then, click Choose.

TIP: You can view more file formats by holding down the option key on your keyboard when clicking the "Format" drop-down menu.

Outcome

You have successfully converted your image files.