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 dimentions are at 800 or higher you should reduce. Pick the highest of the two numbers and change it to "500". The other number will change proportionally. Then hit the "Scale" button.
7. Select the "Update" button on the right side of the page.
8. Close this tab and return to the original "Edit Gallery" page where you started.
9. Repeat for any images that need to be resized.
10. 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.