Transparency of png and gif

[mcxxmc]

I think it is a good idea to add back the transparency of png and gif. Now the gif does not seem to display the border of the nodes, leaving the place all blank.

[mcxxmc]

Hi professor @gallettilance , can you provide more details regarding the bug when transparency does not work? (e.g. the test case)

[gallettilance]

This is what I observed: #15

I'm worried there is a dependency that is changing / not fixed