Usually it's one of the pictures that gets me going, and wedging in the others that takes the time.
Finding those common threads that spark ideas and connections in your mind seems to be the trick. I usually look at the pictures for their similarities first, then go from there. And usually 2 have a decent connection, sometimes the third has a...bit of a connection. But invariably the 4th (and sometimes 3rd) have only very ephemeral similarities.
The beach picture and the shark-gull picture, for example, go together nicely, because that is where gulls are and sharks are nearby. The tree-bee-thingy...well, there is a bit of flora on the beach picture, and I am from a small town in Alaska on the ocean, with mountains and forests going down to the water, so that...can fit. The foot? Well. Hm. It's. Part of an animal. The shark-gull is. Kind of an animal. The tree-bee-thingy is. A. Being. I guess.
So, there ya go, all the inner links I need to write a story.
Personally, I thought my set of pics was moderate level, but I thought...hold on I have to go check the pics again....
match 1 - not similar at all (the bird and the water/boat go together maybe), but not radically different (like a spaceship and a scene from Sparticus) - moderate
match 2 - disassociated AND various genres (trippy mind/sci fi and trippy fantasy) - difficult
match 3 - not similar but seems rather same genre (fantasy-ish) - easy
match 4 - disassociated but not forcing different genres/times/planets - moderate
match 5 - some parts seem to fit very well (old guy looks similar enough to be same guy) and some see very disconnected and difficult (woman with antlers) - moderate
match 6 - disassociated but not forcing different genres/times/planets - moderate (maybe on the light side)
So, I found them all in roughly the same ballpark, though I thing match 3 pictures had a more general theme and match 2 pictures has more diverging themes.
Note this is not a criticism on the judges or any participants - writing styles and finished product are not necessarily a product of the difficulty of pictures. I think the harder the pictures, the more the writer has to concentrate on melding the story around them and the less time to touch it up, and the harder it is to use all pictures eloquently, but the more chance there is for creativity and thus 'wow' factor. Kind of how gymnasts and ice skaters and horse-jumpers and divers and rodeo riders get more points for more difficult feats.