There's often a significant delay (3-4 seconds) before the larger image appears. If the spinner isn't there, it is confusing for viewers.
Possibly related to bug 56374
Sidenote: Is the image loading progressively (as it should) or is it being withheld from view until 100% downloaded? I almost looks like the latter... If so, see #1 here:
http://www.codinghorror.com/blog/2005/12/progressive-image-rendering.html
Version: unspecified
Severity: normal
This is a must-have before wide deployment on all wikis. It's pretty frustrating to not know whether or not the image is being loaded. I would also remove the small image, because it's confusing, and only show the loading spinner by itself.
Why is this taking so long anyway? I thought we discussed loading only the file size that most closely matches the user's screen size, and using that both for the lightbox view (slightly reduced) and the full screen view. So if we use that approach, we should be able to instantly show the full screen version, whithout having to load a separate file size.
I think the spinner shows up consistently now. As a separate issue we could look into interlacing, but I don't think it's really necessary for now.
As for general loading speed issues, that's a core problem and not something we should worry about for the time being.