Fluxus has been closely associated with nearly all media forms over the years. In many ways Fluxus formed the foundation of multi-media art in the 20th century. Fluxus is after all synonymous with the term "intermedia". Interestingly, Fluxus has never been closely associated with the most traditional of all artistic media, painting.
I suppose that part of the reason for so little Fluxus work to be constructed using traditional painting is that Fluxus arose as an anti-art movement (by some definitions) and thus painting on canvas was rejected by many Fluxus artists. Fluxus has also been concerned with the theory and technology of intermedia (the intersections of different media) which would tend to preclude the use of any single dominant medium such as painting. When painting has been part of Fluxus, it has usually been as a constituent medium in a mixed-media work. Shigeko Kubota created her "vagina paintings" in the 1960s for example but these works were primarily perfomances in which the resulting painting was of secondary evidentiary importance.
But there is really no compelling reason that painting can not be Fluxus. One artist that has been making paintings which are both Fluxus and "traditional" paint-on-canvas is Cecil Touchon. Cecil's "post-dogmatist" paintings often use collages as thier models. He will assemble a collage and then "paint a picturre" of his collage using painstaking trompe l'oeil techniques. The resulting paintings are simultaneously beatiful, visually fascinating, and conceptually rigorous and interesting. His work can be seen on his website at http://cecil.touchon.com/.