Glue for indoor mosaics: I use Aleen’s tacky glue.
Glue for outdoor mosaics: Weldbond online at www.weldbond.com also at Hardwick’s in Seattle: 4214 Roosevelt, Seattle 98105 / 206-632-1203
For mosaics on cement or mosaics in standing water (bird bath or shower surround) use thinset morter. Thinset holds heavy tiles in place (prop very heavy tiles in place using a board) on a vertical surface. Premixed Thinset is easiest to use, sold by the quart and gallon at your local hardware store.