One word: Flakey colo profiles. Well that's 3 words, but it is most likely the cause of this since it affects hardware accelerated decoding of JPGs in Windows itself as well. Have them reset their graphics configuration, make sure everything on your end is clean as well and you're not emebdding bad profiles...
Generally we don't embed profiles in JPGs destined for onscreen use. (What would a "bad" profile be in this case?)
These particular clients aren't terribly tech savvy, so I suspect they'll draw a blank here. (And we're not PC users, so it might be hard to walk them through it.)
Any simple advice for them? I'm 99% sure that something changed at their end.
JPEG files are the same, regardless of platform.
But if someone transmitted the resource fork (containing just thumbnail data) by mistake, it might do what you describe. Do the filenames start with a . , _, or % ?
Thanks, but no, the filenames appear to be valid. And client has re-sent to us the "glitched" JPGs — which appear perfect when [re-]viewed again at our end. (And they are the same [large] size when sent back to us.)
Are you using a program like iPhoto where it hides the image in a folder and then just presents the thumbnail? It sure sounds like the PC is viewing a thumbnail, and perhaps on the Mac it is referenced to the real image?
No, we're using Photoshop. (And we've also created the JPGs in InDesign as well, with the same result.) The screen grab sent us by the client shows clearly that it's not just an enlarged thumbnail (which would be pixelated or blurry) but a JPG with the distinctive artifacting around sharp detail (particularly type) which results when the JPG has been created at a very low quality setting — or saved and re-saved a few too many times. This is a large file saved at maximum quality, so it's clearly being rendered incorrectly on the PCs. (Note that when they send it back to us it has that same large file size and, as viewed on the Mac again, the same high quality.)
Again, the files are the same, and Photoshop hasn't changed it's JPEG code in a long time.
Somewhere, there's something going wrong that you aren't telling us about.
My only guess would be a third party plugin that is reading JPEG instead of Photoshop.
Okay, good to know that Photoshop's JPEG code hasn't changed.
Really I can't think of anything at our end (on the Mac) that we're doing differently or that's changed in our hardware or software.
What I can't know for sure is exactly what's happening on my clients' PCs.
What questions should I ask them?