I find that a large percentage of my note taking involves saving photos. Whether it's a business card, menu, bottle of wine... whatever. It's a quick and easy way to capture the information and usually the title is sufficient for search parameters.
For me personally, I am 99%+ of the time on my mobile phone taking the picture. Therefore, implementing in the iOS and Android clients would create efficiencies for many people, including me! Besides, if you're editing photos on the desktop, you'll likely want a more complex edit with more complex software. IMHO, it wouldn't be too difficult to open a photo editor on the desktop and save back to Joplin.
So... while I understand I can edit outside of Joplin, it would be very time-saving to have a built-in photo editor on my mobile client.