1. Add your images to the gallery as you normally would.
2. Select "Add or edit images in this gallery" so you see all the images you've loaded in the "Edit Gallery" page.
3. Select an image so the "ATTACHMENT DETAILS" are visible on the right side of the page.
4. Select "Edit Image" in the "ATTACHMENT DETAILS" area. A new tab will open up in your browser.
5. In this new tab you'll see your image and the words "Scale Image" in a box to the right of the image. Select "Scale Image"
6. You will see some new information appear, including the "Original Dimensions" of the image. If any of the dimensions are at 800 or higher you should reduce. Pick the highest of the two numbers and change it to "500".
7. When you initially put in the new image size "NaN" will appear in the other dimension.
8. Look in the lower left and select "cancel."
9. Then select "Edit Image" and it will again take you to the spot where you can update your image size.
10. Make your size update then select the "Update" button on the right side of the page.
11. Repeat for any images that need to be resized.
12. FINAL THING TO DO AND IT CANNOT BE MISSED: Hit "Save and close" in the bottom right of the "Edit Gallery" page then hit "Update" on the right side of your "Edit Post" page.