It is difficult to look back on something in retrospect and decide how obvious it was at the time. "Nonobviousness" seems like a far too subjective and difficult to determine way to decide whether something should be patentable. What seems like an original, genius idea to one person may be glaringly obvious to another, or may even have already been previously recognized by others.
I think that photomosaics are not nearly nonobvious enough to be patent-worthy. Regular tile mosaics have been around for a long time, and I'm sure people have made mosaics with patterned or textured tiles rather than a single color. It seems like it would be a pretty easy leap from that to making mosaics out of pictures, and then to making software to automate the process. In addition, the concept had
already been used by Joseph Francis several years before Robert Silvers patented it. So even if it had been nonobvious before it had ever been used before, Silvers could have seen photomosaics and then written his own software.
I'll freely admit I don't know very much about patents and copyright, but I think it would have been better for Silvers to copyright his particular program rather than patenting the idea of a photomosaic.