I have done exactly that, twice. Same problem occurs, and what's more I have uploaded silverstripe to another domain on my reseller account and the same happened there too.
I have even tried to delete the images, then re-upload under and different name and then rename them back to the original name but still to no avail.
Really wierd problem, I am almost sure this must be something to do with my server rather than silverstripe but just don't know what to do about it. I have contacted my provider and will await to see what they say.
Thanks