The 'field' (id3 tag frame) name is APIC.
<Header for 'Attached picture', ID: "APIC">
Text encoding $xx
MIME type <text string> $00
Picture type $xx
Description <text string according to encoding> $00 (00)
Picture data <binary data>
You can find more details here: http://id3.org/id3v2.3.0
Have a look at 4.15 Attached Picture