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A summary of the steps are as follows:

1. Open Photoshop and choose File > Open.
2. Navigate to the image file you want to optimize and press Open.
3. in the top menu choose Image > Image Size.
4. Make sure the Contrain Proportions button is selected, then change the highest value in the Width or Height dimensions box.
5. Usually 640 pixels is a good high value but you may want to vary this depending on the image and context.
6. Press OK and Photoshop will resize the image.
7. Select File > Save for Web & Devices. This will open a preview window that will show you what the image output will look like.
8. Select an image format. The choices are GIF, JPG, PNG and BMP. Usually JPG is the best choice.
9. Move the Quality slider to find a compromise between quality and size. The preview window will update automatically.
10. Press Save and put the file somewhere ready for upload to WordPress site or image gallery.

This is the optimized image used in the video tutorial.