It is true that "Lots of more modern arts hang without frames." But the Signac paintings are 100 years old. And even contemporary art in a Wikipedia article benefits from frames in their presentation to the viewer. Virtually any image benefits from lines echoing the delineation of the (usually rectangular) image. This is somewhat similar to the way scare quotes call attention to a word or phrase. The scare quotes are setting off that word or phrase for special consideration. We have a vested interest in trying to get the reader to take special notice of each image of artwork that we include in an article. If we are going to argue that Wikipedia is not an art gallery or that these thumbnails are hardly impressive in their dimensions and scarceness of detail, then why bother including images of famous paintings at all? I think we all agree that the images are all educational in an art historical kind of way. Therefore it seems like a poor move to reduce their usefulness to the reader by removing the framing lines and the emphasis those lines impart to the image.