The term 'indecent image' does have a definition in law, but it is also valid everyday language that can be used to describe something shocking or unseemly.
Viewing pornography (images) in public is both shocking and unseemly (indecent).
Well yes, as I said, and as you have said, it's the context that's indecent, not the image per se. Therefore, the term "indecent image" is incorrect here. It would be more appropriate to say "images that are indecent to view in a public place."