Please make sure to take the photo under natural light. Bring your painting outside your room to the patio or backyard when the weather is fair. Take the photo of the painting with full frame. Portions of the frame is not cut off or blocked by any objects. Your hand should be very steady when shooting the photo.
Here are examples of bad photos
Photo was taken with flash light. The flash light bounced off from the smooth surface of the painting to the camera lens. The frame is cut off -Bad.
The painting is under direct sun light. There is shadow on the frame and painting is harsh. You can take much better photo in the shadow.