In a nutshell:
Batch Resize Images in Mac OS X
You will need to launch Preview, which is in the /Applications/ folder, but is also usually the default image editor and can be typically opened by just opening any picture. Once Preview is open, proceed:
- Select all the images you want resized and open them within Preview
- From Preview, select the images that you want to batch resize from the drawer (Command+A will select them all)
- Now, go to the menu labeled Tools, and then Adjust Size
- Enter a value for what you want the new width and height to be
- Next, navigate to the File menu and click “Save All” or, to save a NEW version that is resized, choose “Export Selected Images…” or “Save As”
The images should be showing smaller file sizes in Finder.
Tip: Create a backup of the folder you will be working with to preserve the originals.