I rotated a few images on my computer, however, image resizer keeps using the old orientation. When I navigate to the image just using the image path, it shows up fine. However, when I add ?h=400, it's using the old orientation. What's causing this?
The application you are using to rotate images is only setting the Exif Orientation metadata flag. ImageResizer will honor aforementioned metadata if you install the Autorotate plugin and specify &autorotate=true in the command string.
Browser and mail reader support for Exif Orientation is extremely spotty; I'm surprised your browser actually rotated the image.
Keep in mind that rotation is somewhat expensive; it's good to have your images actually rotated ahead of time, as it will make images render consistently and a bit faster (especially on mobile or slow connections).
ImageResizer does not output the exif orientation flag, it always produces a true rotated image.