I know that Mac people use the Photos App for photo editing and interaction. However, for down and dirty batch processing of photos use the Preview App. There are other third party Apps that will batch process, but if you do not wish to spend the money and only need basic processing, then use Preview. Here is how that works.
The preview App is capable of processing a photo into several different file formats: GIF, ICNS, JPEG, JPEG-2000, BMP, Microsoft Icon, OpenEXR, PDF, Photoshop PSD, PNG, SGI, TGA, and TIFF. For purposes of this demonstration, I am going to use three photos in HEIC format and change them into JPEG’s.
The first thing to do is open several photos in Preview:
Then, Shift-Select all the photos together. Next, go to the File menu and choose “Export Selected Image”:
This will open a window similar to this one:
Click on the “Options” button which produces a drop down to select the image type for conversion:
If you hold down the Option ⌥ key, you will see even more image conversion choices. Once you select a file format, just click the “Choose” button and the images are saved into the format and folder of your choice:
In this case the HEIC images are converted into JPEG format. The Bulk Export Images feature in Preview is supported in all versions of Mac OS, including macOS Catalina, MacOS Mojave, MacOS High Sierra, Mac OS Sierra, Mac OS X El Capitan, Mac OS X Yosemite, OS X Mavericks, Mountain Lion. To be quite honest, I don’t think I ever noticed it before now.
You can use third party Apps with lots of export features to bulk convert images, but if all you need is a quick conversion for an Email or some other reason, just use the Preview App.