Summary: (from others and my experiments)
(1) Movie covers seem to require a rotated Aspect Ratio (rAR=y/x, aka portrait) between 1.33 to 1.78? Actual pixel counts don't seem to be the issue...
(2) Series seem to require square covers, again actual pixel counts not critical. Portrait artwork is ignored in iOS 8. If no cover artwork, a snapshot from the content is used... Episodic snapshots still not generated!
(3) Music videos - only had a single problem. Resolved by re-compressing with dimensions divisible by 16... Don't know if it was the re-compression or the dimensions that resolved the problem...
(4) Keep a pre-12.0 version of iTunes around - it seems you'll need it!
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Requesting Apple to Fix: (in priority)
(1) TV series cover artwork should be allowed to be portrait!
This is crucial - I use series for Documentaries, they always have portrait (DVD cover) artwork;
Others have expressed the desire to use their better DVD artwork!
(2) Individual Series episodes should still get a snapshot in the entry
(or at least it should be published how to re-enable episode artwork);
(3) Rules for movie cover rAR should be loosened again...
(4) I do not plan to update iPad to iOS 8 until this is further resolved
(and probably MacBook Pro to Yosemite) Please FIX IT.
Anything missing?
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Details:
Early in 1st gen iPhone I got stung with an update that corrupted my Microsoft Office Outlook DB. Took many hours to fix! Learned to wait a while... Last weekend finally upgraded one device (iPhone) to iOS 8.1.2. Although disappointed in certain features still missing, the only major problem so far is the video covers, and found your forum comments very helpful! Here's what I think is happening:
Movies:
I noticed that most failures were old B&W movies... The cover art was often of 'video tapes' - tall and narrow artwork. Led to study rotated Aspect Ratio (rAR) as the problem...
covers that fail: pixels(x,y), rAR (y/x), notes
201x364 1.81 AC {resized to 216x364, rAR=1.68 works}
259x475 1.83 DW {resized to 272x475, rAR=1.74 works}
500x500 1.00 FH {cropped and resized, works}
256x475 1.85 Fa {resized to 272x475, rAR=1.74 works}
some odd sized covers that continued to work in iOS 8:
334x475 1.42 Ch
90x159 1.76 JSO
356x500 1.41 Ju
350x500 1.43 RE46
So, no specific pixel sizes for the covers, but rather, Aspect Ratios...
Seems rAR between ~1.33 to 1.78 are safe (1.41-1.76 known to work).
Note: sometimes looses all covers in process - you need to completely reSync, sometimes several times! (This is why I use a weekend!)
iTunes 12.0 some day may be a better step forward, but not yet.
Keep a pre 12.0 version of iTunes around if you can!
It seems iTunes 12.0 doesn't allow reloading an adjusted cover artwork - it seems to always keep the old artwork - even when deleting the video from the library and reloading it. To fix it:
0) Delete the video from the iPhone.
1) Delete the video file from the 12.0 iTunes.
2) Used a iTunes 11.3 (on Windows for me) to load the video and delete the cover.
3) Add the new cover in iTunes 11.3.
4) Reload the video into the iTunes 12.0.
5) Change from Home Video to Movie.
6) Download to iPhone.
Eureka, the cover is fixed. did this for the problematic files, with adjusted AR cover art... All worked.
TV Series:
TV series are too transient - I leave them off iOS devices, because iOS devices are very difficult to load.
(hint, android much easier here - but no pretty covers...)
(Still, I choose both platforms to access external servers of media for daily consumption).
Instead, I place my favorite documentaries. Good documentaries often come as a collection of videos, so Series constructs work well - but choice of metadata is crucial)...
Clipped failing artwork of 332x500 px to 332x332 > re-attached > artwork shows-up in Series tab!
If no cover artwork is attached, a snapshot within the video is attached at the series level.
Episode snapshots still missing.
Tried a second, sample image in artwork of file (as episode image), no luck...
Have decided not to attempt to 'fix' series (documentaries) images -
this is definitely an Apple failure and it's far more important for them to fix it on their side!
Hope this helps you resolve movie covers.
I think Apple should be held responsible to fixing Series covers!