I doubt they could have told you!
My guess is that the function to insert a custom emoticon is not checked for dimensions or goes through a different API package. Either way, the methods don't appear to be the same.
In any case, the method in which you suggest is counter-intuitive anyway. I'd be surprised if it were somewhat discovered in testing (I wouldn't expect many or any other beta testers here to have picked that up), and then it wouldn't be raised anyway as the development team would know that is certainly not the most intuitive way to upload a photo.
In fact, having the option to upload a custom emoticon doesn't really make sense. If someone wants a custom emoticon, it would be an image anyway. The purpose of emoticons is to have a predefined set of small, accessible icons, and usually are rendered through text symbol replacements.