[aerobson]

It's a best practice because it gives the user as much possible as soon as possible, and because there are file size savings.

When you say

it would be better to have a placeholder there that is the same frame as the final image size, then have the final image appear upon completion

isn't that the quality of progressive jpegs?

[weixiyen]

I think the main issue I have is that if there is a situation where you actually notice the paints happening on a progressive image for it to even matter, you might want to rethink the level of quality of the photo being delivered to that device over that connection speed.

The way I'd approach photos is this - no image should take more than 3 seconds to load, and anything between 1-3 seconds has a small progress bar (mobile) or just an empty frame with a thin border (mobile thumbnails or any web photos).

I'd take into account device, connection speed, CDNs, jpeg compression to ensure that I meet the time requirement for the full image to load.

If the full image isn't consistently loading within that timeframe, I've already lost and need to rethink the quality of the images being delivered or if I'm designing the right app / site, because it's going to be a terrible user experience either way, progressive jpeg or not.