Put copies of all the images you want to resize, irrespective of their sizes, into a new folder. Name the folder something like "Thumbnails" even at this early stage.

Make sure that all the images are in a web-friendly format, such as jpeg or png and set to 72 pixels.

Activate Automator.

If you have never used it before, Automator is in your Applications folder. Double click on it.

In Automator, in the left column, Library, click on applications, and then on Preview.

In the right-hand column find "Scale images" and drag it into the main window of Automator. You will be asked if you want to protect your image files by creating a copy of them. Click "Add". It is better to be doubly safe.

Position your Finder window and Automator so that you can see your "Thumbnails" folder. Select and drag your images onto the Automator window. They will be listed a new frame called "Get specified Finder items". This new frame should be placed above the "Scale images" frame.

Set the amount to which you want to resize your images (in pixels, for the longer side.)

Now click the "Run" button at the top of the Automator window.

If you watch your Finder folder, you should see each file changing as it is rescaled (they are no longer highlighted), and by clicking on the individual files, you will see their new sizes in the preview column.

All the files in your "Thumbnail" folder are now rescaled.

The beauty of this method is that it recognises portrait and landscape formats and treats them intelligently. You may want to find any square or nearly square images and resize them by 90%, so that they have the same visual size as the more rectangular ones.