In my experience, most of the glossy printers that drugstores (and the Walmarts of the world) use for photos are 300dpi equivalent or greater. Until you get your source information to that resolution, you're unlikely to see much improvement by changing printing processes or papers. Different sized images can result in different printers being used, with different results (e.g. a 36" wide print is likely to come off the wide-format inkjet printer, while 3"x5" prints are likely to come off the dye sub photo printer - hardware and process varies widely, though).