I was looking into using Shortcuts, but wasn’t able to figure it out.
It’s so frustrating because the Mac comes with the tools to do this via the Preview App, but on the iPhone and iPad, much of the Preview app is backed into the OS, but converting one file type to another isn’t.
You would think it could be handled via a shared sheet. A save to images option when you open a PDF.
Unfortunately, a lot of services allow you to post images, but not PDFs.