Feasability of module or submodule for user picture migration
I hadn't realized that the dev versions of migrate and migrate_d2d now support migrating user pictures.

The UI asks for the location for files in the installation one is migrating from. In my case, pictures were stored under files/pictures but I only had to specify "...files". I don't know if it would work if user pictures are stored in a directory with a different name.

In any case, once I switched to the dev versions, it seemed to work.