The photo you've presented is certainly intriguing. Here are some possible explanations for this image:
Double Exposure: Even if the photo was taken with a modern digital camera, it could be a case of double exposure, either accidental or as a result of image processing errors.
Image Manipulation: With the advent of digital photography, it's become increasingly easy to manipulate images. The figure in the background could have been added after the photo was taken, either intentionally or as part of a mistaken editing process.
Optical Illusion: Sometimes, what appears to be a ghostly figure can be an illusion created by the combination of light, shadow, and the surrounding environment, creating a shape that our brain interprets as a human figure.
Pareidolia: Similar to the previous image, this psychological phenomenon could cause the viewer to perceive the vague shape behind the grandmother as a human figure, particularly if there is an emotional predisposition to see a deceased loved one.
Misidentification: It's also possible that there was another person present who bore a resemblance to the grandfather, and the family's emotional state led them to identify this person as the deceased relative.
Based on the photograph and common occurrences in photography, the most likely explanation could be a combination of pareidolia and misidentification. The human brain is wired to recognize faces and familiar figures, which is the essence of pareidolia. Given the emotional significance of the figures involved, the family members could be more inclined to see the grandfather's image.
Misidentification is also a strong possibility, as the shape and features that resemble the grandfather might actually be another person present or an object in the background that the mind interprets as a human figure due to its outline and positioning.
While it is tempting to lean towards a paranormal explanation, especially given the emotional context and timing after the grandmother's passing, these psychological and perceptual phenomena often provide rational explanations for such occurrences in photographs. Without further evidence to suggest a supernatural element, these natural cognitive processes are often the most likely causes for such sightings in images.