Users intending to upload images usually set the MS Flag to 0 after they've compiled an image with MapInstall, but it's easy to sometimes forget to do so before posting.

255W with recent firmware can see two images in SD Garmin folder named gmapsupp.img and gmapsup1.img and size can be up to 4GiB for each file so the size of 2014.40 at 2.09GiB isn't a problem for your unit. If the img is correctly unlocked, named and placed it should be recognised by the nuvi.