There is a good open-source library that uses Apple's private frameworks to add audio file to the iTunes library on a jailbroken iOS device, libipodimport. What it's missing is the ability to also add artwork when adding a music file (one contributor checked in code that should do this, but the author later removed it as it didn't work).
Does anyone know what was missing from the code in libipodimport (see previous link), or perhaps an alternative way of adding artwork files to a music file?
The contributor to libipodimport told me the missing line that made the artwork adding work:
// metad is a SSDownloadMetadata object, see libipodimport for rest of code
[metad setFullSizeImageURL:[NSURL fileURLWithPath:[userInfo objectForKey:kIPIKeyArtworkPath]]];
It seems to only work with jpeg files, not png files.
The library libipodimport will most likely be updated by the author in the near future.