Update: I've added a new section to the main page of this group, listing the type of screenshots we need at the moment.
Alright, thank you all who offered your suggestions (or even that you're around and paying attention), I've noted and contemplated each of the recommendations. In the end, I've decided to go with an imageboard for now. You can find it at:
This is where the scouts will post their findings. Of course, I don't expect them to upload hundreds or thousands of images each time, a few images can be sufficient if they show the thing we need well enough. I've uploaded one screenshot as an example, note that the tags should reflect the contents of the image in an almost absurdly detailed fashion: "adult, mare, three-quarter view, talking, standing, indoors, door, cabinet, furniture"... this will make it easier for artists who to find the particular things they need for reference a lot.
So, let's start with what we need for the next couple of comic pages. I may or may not have mentioned already that this chapter will be BonBon's "origin story", going back to her fillyhood in Marehattan. So we will need screenshots depicting the streets of said city, the pedestrians roaming the streets, and indoors (homes, shops, if there were any shown in the show). Also, we will need images of fillies (different views, emotions, etc). One way to find such screenshots is going through existing repositories (I looked up on the mlp wiki galleries years ago, please let me know if you know any good such mass screenshots repositories and I'll post it on this group's main page), or watching the episodes you presume have any and make some yourselves (I used to use UMplayer, it's easy to make screenshots with it).
(And like I said, we don't need hundreds of images depicting the very same scene, a few can do the same. And needless to say that all images should be screenshots from the show, and thus be of the SFW kind. The screenshots should also be of acceptable quality, 720p is ideal I think (but 480p is acceptable too)).