Resizing Multiple images at once on a Mac using Preview

Create a folder with all your large images in them. 
Right-click and choose "Duplicate"
Rename that folder "Thumbnails"
Now open that folder.
Hold down the Command Key and "A" to Select All the photos
Now Right-click and choose "Open"
Once Preview is open, Click on the first picture in the right sidebar.
Now hold down the Command + A again to select all the photos in the right sidebar
Click the Tools Menu
Then click Adjust Size
Now in the Width field, type "150" (or however wide you want your thumbnails to be).
In the Resolution field where it says "multiple values" type "72" (that is the number for web resolution).
Click OK.
Now click "File" >> Save All

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