There is likely some risk that you could be sued for copyright infringement, at least for the pixelated Marilyn Monroe image, which is likely still under copyright.
I believe the other image is a pixelation of Girl With a Pearl Earring, which is a 17th century Vermeer painting and, thus, likely outside of copyright. The complication there is that a movie was made "inspired by" the painting (or actually made out of a novel inspired by the painting), and the copyright owners there might claim the image is derivative of a movie poster or scene (which would be under copyright) instead of the original painting.
Pixelation of a work could be considered a derivative work, and thus copyright infringement. This was certainly the case in 2010 when Andy Baio was sued by photographer Jay Maisel over a pixellated version of the cover of Miles Davis' Kind of Blue which was a photo shot by Maisel. Baio settled out of court, so there was no ruling. But Maisel's suit shows that there is some risk to this sort of derivation.