This has been issue for long time even for iPods before iPhone.
The cause is this.
There are multiple ways of manually applying artwork to a song in iTunes. Only 1 is the best way to make sure artwork goes with the file.
The three ways are:
1. Drag a file/image to the bottom window where it displays artwork when a song is selected.
2. Using the "Find Album Artwork" option in the right click menu for a song.
3. Using "Get Info" option to bring up the Tags of the song and pasting a image into the artwork tab.
The 3rd is the best and only option you should use.
Here is why.
If you use the little artwork window and put images in there, or if you tell iTunes to find artwork, what it does is uses the image supplied or the one it finds and then saves that image to a hidden file on your computer. Then iTunes notes/saves a pointer to that file matched to song. So "iTunes" when playing or looking at the file will say, oh there is a pointer to a file, let me go find that file and display the image. All good and dandy for iTunes.
However when you move that file (manually to another machine or sync to a device) it just moves the media file. The hidden image doesn't go with it as iTunes just uses it.
So the 3rd option of using "Get Info" to get into the tag editor does it different.
In the tag editor you are literally editing data contained in the media file itself. Thus if on the Artwork tab in the tag editor if you paste an image in there, when saved it will
embed the image into the media file itself. Thus if you move the file to another computer or sync, that file contains the artwork thus will always be available to it no matter where it is moved.
So in summary, ONLY ever add artwork by using the ArtWork tab in the tag editor (Get Info option).
Nice thing is too when you do, since it literally changes the file (can see date change property update...and filesize) then when you sync it will notice a file change and update the song on the device. If you use the other options, no change is ever done to the physical file thus you will notice the file not change it's change date or size, thus on syncing after assigning artwork those other two ways, it will not retransfer the file because to the iPhone/iPod, it has not changed so there is no need.
Message was edited by: DaVBMan