I would say this strongly depends on the usage case. For example, if you have a profile picture, I would recommend uploading it in 2-3 versions, as those pictures may be downloaded very often (for example in a social networking app where you have profile pictures in posts, user profiles, messages, etc.). When the picture is downloaded a high amount of times, you would rather download a smaller one to minimise download time and save parse data transfer resources.
On the other hand, for pictures that aren't downloaded as often as other ones, I'd recommend storing them in a high-res format, and scaling them down (if necessary) as they are downloaded. Take for example again a social networking app. A post contains a profile picture (which is downloaded quite often) and the actual post (a photo in this case). The actual post photo is only downloaded a single time (ideally), so there should be no reason to worry about the download speed.
Basically (and that's the way I handle this), you should always try to cache every image. Images that can be cached easily and don't have to be retrieved very often can be stored in a single high-res format (saving data space on parse). Images that cannot be cached easily or have to be refreshed quite often, should be stored in different sizes, which will, in the end, save you data transfer. The small amount of extra storage does not have that much impact, to be honest, especially if you store them in scaled down sizes.
