Add 3 warnings of pixeled image

[PedroCuadra]

I made 3 warnings for pixeled images. I hope that will be useful

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[ebidel]

Thanks for the PR. I'm not sure we'll take this b/c the icons in this collection are actually generated from the Material design spec: https://github.com/google/material-design-icons

There's a script that pulls the master icons: https://github.com/PolymerElements/iron-icons/blob/master/update-icons.sh

[addyosmani]

I'd recommend opening an issue on our Material Design icons repo Eric linked to suggesting these icons. It's possible the design team are already working on something similar or may be interested in doing so.

[notwaldorf]

Closing this, as @addyosmani and @ebidel are right -- we automatically generate the icons using a script and the Material Design icons repo, so your icons would just get overriden next time we run the script.