I tagged a file in Picard, but iTunes is not seeing the tags!

First, you need to force iTunes to re-read the information from your tags and update its library. This is discussed in the iTunes Guide.

Additionally, iTunes has a known bug in its ID3v2.4 implementation, which makes it unable to read such tags if they also contain embedded cover art. As a work-around, you can configure Picard to write ID3v2.3 tags.