SoundCloud's Visual Player is the default on Tumblr to include the artwork for any track or playlist that you share.
You can share to Tumblr by clicking the 'Share' button underneath the track's waveform and then clicking on their icon to send it out to your Tumblr audience.
Manually posting to Tumblr
You can manually share to your Tumblr by copying the track or playlist's URL and pasting it into the Audio section when creating a new post.
Tumblr also allows you to use tags to help users discover new tracks. ‘Music’ is a good place to start; include your genre, name and the subject matter of your track or playlist, too.