Since the 'id3tag' utility included with id3lib does not support APIC frames, I decided to write my own command-line utility for manipulating attached pictures. I am hereby donating this utility to the id3lib project and do hereby release any and all claims of ownership or copyright covering it.
This utility is written in C++0x and depends only on standard C++ classes, id3lib, and the GNU getopt_long function. You should be able to compile it using the following command line:
g++ -std=c++0x -O3 -o id3pic id3pic.cpp -lid3
The utility supports listing attached pictures (with their picture types, MIME types, descriptions, and binary sizes), removing selected pictures by index, exporting selected pictures by index to external files, and importing pictures of user-specified type from external files. The utility accepts multiple MP3 file names on the command line in all cases except when exporting (since that would result in repeatedly overwriting the exported image file(s), which would be pointless).
My intent is for this utility to be adopted into id3lib and to be distributed alongside the other utilities such as id3cp, id3info, and id3tag.
id3pic.cpp