2.8MB shouldn't be too big, actually. The site will accept a photo of up to 24MB in size.
I wonder if your reducing the photo in size actually also converted it to a cleaner JPG.
iphones and other apple products are notorious for doing non-standard things to their photos and this makes it difficult for home-grown software (such as what runs Garden.org) to accurately handle them. That's likely the case there.
I'm getting better at handling photos though. If you run into the problem again and are willing to help me troubleshoot it, shoot me a tree-mail and I'll be glad to let you know what you can do to help.
