I was able to do 2 on Friday. After completing them, the rest of the available shops disappeared from my map. But my Friday shops were approved (and paid) today (Monday). After I was notified that they were approved, the rest of the available shops reappeared on my map.
I was able to do 2 more today, signing up directly on the Presto app. It seems like the 2-at-a-time limit might be gone now, since I can still see (and potentially sign up) for the rest of the available shops in my area, even though I did 2 today (Monday) which are still waiting for final approval. But I'm pretty sure a good portion of the locations listed in my area are still closed for in-store visits (they are drive-thru or curbside-only right now), so it wouldn't be possible to conduct the shops for those locations at present.
In regards to the photos required...I think it depends on how you answer the questions on the survey. If you answer "yes" for some questions, they will want a photo documenting the applicable signage/markings/etc. For example, if you answer that there are directional and social distancing signage outside the location/building, you will be asked to provide a photo of that. But if there are none present (and so you answer "none" to the question), you won't have to upload a photo (since there would technically be nothing to photograph).