The general problem with the use of ephemeral keys in this context is discoverability. Simple events signed by ephemeral keys will probably vanish into the void. Also, since ephemeral keys are cheap to produce, how do you prevent disinformation campaigns from flooding the system, making truly relevant content hard to find?
I feel like PoW is a good answer for the latter. While a public channel for "reports" implemented with gift wraps could address the first problem.
Also you have to be aware that even with all the precautions opsec must still be practiced by the source. A picture can easily leak their location and put them in harms way for instance.