How to use the "instant alpha" in Preview
The way Instant Alpha works is that it selects color that are similar to it.  It's not as nice as Photoshop where you can select multiple areas and get really granular with the selection criteria so you'll have to do multiple steps.


Select the Instant Alpha tool
Using your mouse, select an area you'd like to remove
Slowly drag your mouse down and select an area you'd like to remove.  If you select too much, back off until you've selected as much as you can
Hit the ⌫ Delete key
Repeat until you're removed the background completely.




(I may have gone too far on the first background selection, but you'll get the idea)