(Sorry if double posting but posted yesterday and didn't see my message)
on en.wikipedia.org :
http://en.wikipedia.org/wiki/Image:Newtroot_1_0_0_0_0_m1.png (actually it's on commons : http://commons.wikimedia.org/wiki/Image:Newtroot_1_0_0_0_0_m1.png) doesn't display, instead the page shows a big gray square with "Error creating thumbnail: Invalid thumbnail parameters"
I've tried on a local install and got the same error.
This image used to be smaller but was recently "upsized" so I've tried to resize the image (5600x 5600 --> 1400 1400 : 5Mb --> 1 Mb) and upload it locally and it worked fine (didn't tried on wp)
Could this be a problem linked either with the size (in bytes) or the size (in pixels) of the image ?
Alexis
(BTW there are several others "Error creating thumbnail: Invalid thumbnail parameters" on en.wp and commons)