I don't think it's your browser. It did that for mine too. I'm using Chrome. Maybe it has something to do with the change in format from older, smaller preview size to the larger one, which occurred many months ago. Looks like the smaller ones didn't update.
(Edit: Never mind my theory, Xueling's images are both newer.)
they change in size sometimes i don't know why that is. i think it has to do with the ratio of the piece. the large one is 6000x8000 and the other --- interestingly the zoom box didn't work on that... so while it might be ratio maybe the small images you can't use the zoom box on (where it just keeps loading).
oddly it's random. i'm in the process of fixing images that may not print, and i had to send a file a few times because i saw things that should be cloned, etc. and one time it was large another time it was small. so i think there is legacy code at work here. it might be a good idea to post it to the bugs thread, mostly because you already have the links convenient.
I could fix all problems by re-uploading the image file.
Of course not from scratch, but via the "Edit" button of the existing image.
In this way the votes and comments are preserved.
Not an elegant solution, but it seems to work ...