I added an avatar which has a white background, and while I don’t have the means to verify, I suspect this area is marked as transparent as part of the PNG file. However, while Spark displays it correctly when you browse to it, upon being loaded the white is turned into black.
This is using Spark 2.5.6 with an OpenFire 3.3.2 backend using the LDAP vCard Avatar 1.0.4 plugin.