Copy the image in an image editor (the image, not the file), then paste. Otherwise, you'll need to rename the image file you are using to exactly match the artist's name in your tags (Elvis Presley.jpg for example) and move it into \Library\Artist Pictures\Thumb in your MB installation. If memory serves, the default image size is 500x500 pixels.
Home Desk ~ MB 3.3 Portable • Questyle CMA400i (ASIO) • Sennheiser HD 660S (balanced) / Audeze EL-8 Closed Back / Fostex TR-X00 Ebony • Teac AI-101DA • Jamo C93 + Dayton Audio SUB-1000
Work Desk ~ MB 3.3 Portable / Tidal • SMSL SU-8 v2 • Nobsound NS-05P • THX AAA 789 • Sennheiser HD 58X (balanced)
OTG ~ FiiO M11 • Audiofly AF180 / B&O H6