You can reduce the glare by placing the picture in an area that doesn't face direct light. Either real or artificial light.
If the picture is not framed with a paper backing you can also go to any local frame shop, or craft store that does framing, and have a piece of non glare or UV protected glass cut to fit the frame and replace the glass yourself. Not only will the UV glass cut down on reflective light, but it will protect the piece from damage.There should be at least 1/8" air space between the glass and the picture as well. This helps to reduce any unwanted moisture content, allowing proper air flow and less of a likelihood that the piece, if of any value memory or monetary wise, will adhere to the glass and ruin the piece.
If that's not an option, indirect lighting as I mentioned first is your best bet in glare reduction.
I hope this helps. If you need anything further, feel free to ask,