Angelino Tweebsly was the famed artist who did the original ceiling mural inside the dome of the Temple of Small Gods in Ankh-Morpork. This herculanean task took him twenty years. By inference this was several hundred years before the consensus present - the power of Hughnon Ridcully's voice causes flakes and shreds of paint to drop off and fall on the congregation. As the god Blind Io points out when considering a suitable punishment for the rash impiety of Leonard of Quirm, by the time of The Last Hero the decoration is in a terrible state. Commanded by his Gods to repair, renovate and generally make good, Leonard finishes the work in three days flat (a nice symbolic number, in religious terms).
Presumably a reference to Roundworld's Michaelangelo, who painted a large proportion of the Sistine Chapel ceiling and sanctuary wall. That took him four years. The outcome was rather impressive, too. Where Leonard is asked to make a complete image of everything that exists on the Disc, Michaelangelo contented himself with representing the majority of Catholic doctrine and scripture in his murals.