Hello Joel,
If your site does not have any "odd" CSS rules, which I doubt it has, Simple Image Gallery Pro will display the image as-is.
This means the image will be shown the way it was saved. It will force its own orientation.
Sometimes image viewing software will save the image when you change the orientation when viewing it (before uploading) which leads to the misconception that the extension might change the orientation.
Finally you can see the PRO version in our demo site.
demo.joomlaworks.net/simple-image-gallery-pro