A mosaic photo is an image made up of a potentially huge number of smaller images so that when you stand closely you can see the small individual images but when you stand back you can see the larger image. They are a fantastic way to display thousands of photos in one large poster sized print and we've done a number of them for people over the years.
There are really three tricks to making a mosaic photo look great. First you must choose the right main photo. A good choice of main photo is a photo without too much fine detail which will be lost and some good variation in colors to allow all the underlying individual photos to be used. Second you need to make sure to select as many photos as you can and if you are in the habit of taking multiple similar photos only choose the best ones for your individual images to avoid duplication as much as you can. Finally and we are a little biased here but choose a printer that can do a high resolution print to insure the detail is visible on the individual images.
That's about it honestly the process is not that difficult and can be done and printed in only a few weeks. If you'd like to get started we would be happy to help.