I don't know if an HTML comment would satisfy the requirement for an image. Then again, MIT is source code license and not designed for images anyway. CC lice

I don't know if an HTML comment would satisfy the requirement for an image. Then again, MIT is source code license and not designed for images anyway. CC licenses are designed for all kinds of things. The designers in this case wanted attribution, so the choice is to either use it and attribute or not use it. It's worth nothing that CC-BY-SA is incompatible with the GPL and similar licenses, so you would not be able to compile the images from Material Design into a GPLed app. You can include them with a GPLed app and load them at runtime, though.