As this picture and caption makes abundantly clear, online is a wonderful medium for rich self reporting using photos and video. While video is considered to be the ultimate in capturing participant experiences, photos can be equally strong in the right context and much more efficient. So which to use when? Let's compare shall we?
Which you use when is a factor of goals for the activity, study and the type participans you are working with. Photos tend to be better for capturing very quick moments like the donut moment above. So moment to moment diaries around usage and consumption are a good match for photos. Video is great for environments and processes - "here's what I do, and where I do it." While cameras are getting easier and easier to use in terms of taking video, participants still have to post the video, which can be a time consuming process, so if you have a lot of activities planned, picking your spots for video makes sense. Also, keeping in mind the technical sophistication of participants is always key in making sure activities match the abilities and availability of your participants.
In the end, it's not an either or decision. Video and photos can be used in concert together within a study to leverage the best of each in bringing your participants to life.