Interesting. A few years back I did quite a bit of data mining/processing, by downloading and analyzing meta data for the top 250 explored photos. At the end my scripts produced daily updated lists of groups which were the most favorable for being explored. At that time, the best strategy for being explored was to submit your just uploaded photo to 1-2 smallish groups (~1-3 thousand members) who were particularly good in cross-liking each others photos on a specific day of the week, and had a limit of one submission per week. (One of them is still around and very active, judging from recently explored photos - Macro Mondays. I even created my own group for this purpose (Surreal Sundays), but it never took off.) The trick was not to like back the person who liked your photo on the same day, or it gets nullified. From my stats, it was sufficient to get ~10 likes in the first 8 hours to make it to Explore. If you submitted your photo to many more groups, your chances would drop. Of course, this algorithm was fairly easy to circumvent - all you need is at least 10-20 people who would cross-like each other photos, but on different days. As a result, way too many yellow buses 
Good thing they are changing the algorithm - at last for now, it seems to harder to circumvent.
