The transparent ones look terrible with dark-themed IDEs or non-light background:

Only looks fine in Eclipse:

Write a program that goes through the images in https://github.com/ReactiveX/RxJava.wiki.git images/rx-operators, checks if the image has transparency, draw it onto a white background then save it as originalName.v3.png (because some of those are linked all around). Find the filename in the source code and replace them.