When a Sprint Health Gadget is configured to show assignees and displayed as a wallboard, then avatars for some users do not appear (they are rendered as 0x0 pixels). This only seems to affect users that have selected from the built-in avatars that come with JIRA. Users that upload custom avatars appear correctly.

See the attachments for examples. The white background is in standard dashboard mode where all avatars appear. The black background is in wallboard mode where some avatars are missing.