You will almost certainly not know the answers to all the questions that are generated, but it doesn't matter as you will not be answering them all.
Pictures you find on the Internet will usually be accompanied by explanatory text, so when learners have generated the questions, make this text available to them.
Now ask them to consider which of their questions:-
a) can be answered immediately (share a few of these)
b) could probably be answered with further research (you might set a few choice questions as homework or as a research task)
c) could probably not be answered definitively with any amount of further research (some of these may be suitable starting points for a Philosophy for Children enquiry)
The beauty of 5Ws as an activity is that the learners have helped set the direction for the forthcoming unit and articulated what interests them about it. Of course, you could use a story, poem, piece of music, artefact, anything instead of an image.