Be careful to at the outset establish that the face of the picture does not stick to the glass, assuming there is glass in front of it (otherwise you probably would not be asking your question). If it does stick then you have a particular challenge - do not attempt to separate the two but instead remove them from the frame together. Once all backing card, frame etc. are out of the way then put the glass and picture together into a vessel of water, glass downward until the picture more or less floats off. Gently retrieve the picture and place on a towel until dry. You may need to press the picture once dry (between two flat, dry surfaces).
Most people would assume that you are dealing with a printed photograph - if you are talking of a framed artwork then the responses will need to be differently put.