It is not easy to produce a good photo of a painting or drawing (framed or not). Experience shows that undesirable reflections often alters the image. Best results are achieved the following way:
Avoid sunlight or direct electric light
Ideally, the picture should be taken under a cloudy sky that casts no shadows.
Electric light often also alters the colours, which get a yellowish tint. Colour correction can however be done by post-processing (I would do that), but it is very difficult to remove reflections.
Be sure to de-activate the flash of your camera!
Position the painting
The light must be totally even. Reflection of the photographer himself must be avoided, even with an indirect light. You can control in the viewfinder of the camera that
a) the photographer casts no shadow;
b) there is no reflection of any kind.
Paintings have often a somewhat glossy surface that is very prone to create reflections. For drawings and watercolours, it is best to remove the protective glass if any.
You can make a few trials by positioning the work and the camera at different angles. A little distortion of the perspective is acceptable.
If you take the picture indoors
As you cannot use the flash, the amount of light is reduced. Therefore, the shutter speed will be slow, and there is a risk of motion blur. Be very steady when taking the picture, or use a tripod.
If possible, it is better to take the picture outdoors under a cloudy sky. There the tripod is not necessary.
Try to avoid perspective effects: the picture should be a rectangle, not a trapeze. However, this can be corrected at a later stage (I would do that) using Photoshop or another good picture processing software.
Framing the photo
Try to leave a little margin around the picture, so that nothing is missing (e.g. the artists's signature), but not too much margin, which might alter the overall colours and contrast of the work.
Small drawings and watercolours can be scanned instead of photographed
Be sure not to damage the work when scanning it. Scanning a framed picture is generally not possible.
When using a scanner, apply a resolution of at least 300 dpi (dots per inch).
Make sure that there is no dust on the surface of the document or on your scanner.
The same applies to scans of a photograph or any printed document.
To scan a newspaper, a magazine or a book, apply the "descreening" option of your scanner, if available. For original works, however (not printed), don't apply descreening.
Size of the digital picture
For the on-line genealogy, the minimum size of the scanned or photographed images should be at least 500 pixels in their smallest dimension. Today's cameras supply pictures of 3000×2000 px, and the best ones 6000×4000 px or more.
If the picture is destined to be projected or printed, it should have the maximum possible size, at least 2000 px wide or high. Scanners should be set at a resolution of 600 dpi or more.
A good picture in JPEG format may thus have a "weight" of several MB (mega-bytes). Caution: some e-mail services do not allow messages with more than 5 or 10 MB.