Using the google page speed test I saw that some images were too large and needed to be resized. I used the ftp to download the necessary images, resized them locally, and then used the media upload tool to import them into Wordpress.
I went to each relevant post and changed the feature image to use the new ones. In the media library the file name is the same as the updated images. However after updating and using the chrome inspect tool, I see that it is still using the old image size even though the visible appearance has changed.
The filenames are confusing me. The original images were named as follows: image-1000x1000. The new ones were named image-500x500. After updating the posts with the new images, chrome inspect is now showing: image-500-1000x500-100. What does this mean? Also, the original size of the photo is still the larger one.
The theme was auto generating alternative image sizes for responsiveness, so my newly named images were actually duplicates.
Leaving this up in case anyone makes same mistake and stumbles across this.
You don't give enough details but your image naming is just bad for wordpress. I am not sure if anywhere in core there is code that tries to detect a resolution by the filename suffix but it is a fragile and confusing habit. If you have to have the data in the name maybe use underscore instead.