If you can take the photo out and scan it then do so. If it's a very big picture you can scan a section at a time, use photo stitching software and digitally put it back together again.
If it's taped into the frame and you might damage the photo then PLEASE don't take it out. We are not in the business of damaging valuable photos! If possible take it to a professional photographer and get then to take a photo of it and give you the image on a disc. That can be expensive so here's how to do it yourself:
Prepare the framed photo:
That basically means cleaning the glass. You don't want a photo of a century of dirt if you can help it. Try a soft glass cleaning cloth and only use a spray glass cleaner as a last resort if the glass is really dirty - the camera/scanner will pick up every smear and that will be worse than dirt. You can digitally remove specks of dirt, it's tricky to remove smears.
If there are small difficult spots of dirt or glass chips that doesn't matter - you can digitally remove those too with the scratch remover or clone tool.
First of all try to get a photo without using flash. Take the picture to a room which is well lit by a window on a bright but not too sunny day. Put the picture at your head height perpendicular to the window. This will give you light but won't let the camera focus on the window.
Bring as many artificial lights as you can into the room. Switch on the overhead light and any floor standing or desk lamps you may have. Don't have them pointing directly at the photo. Basically you are trying to fill the room with light without pointing any of it directly at the photo.
Take Your Shot
Actually you're going to take several! Always take several shots with any photo. The best shot is always the one you could have taken ...
Use a tripod if you have one. If you're standing keep your feet shoulder width apart and relax. You MUST have the camera and the picture parallel to each other or you will completely mess up the perspective of everyone's heads. Stand directly in front of it, not at an angle.
Get ALL the picture in the shot, including the frame. Take several shots!
It's worth taking a few flash shots as well in case they provide something useful or in case the other shots don't work out too well. Now, the problem is you will get a flash burn on the resulting photo, like this one below.