Researchers used a computer-based system to analyze millions of photos for various characteristics and how they may correlate with total views on Flickr.
Where your photo appears within a page of photos (e.g. upper left or middle) can impact its viewership. Of course, so too does the content of the photo.
The presence or absence of certain objects is effective “for predicting popularity”.

“Some of the most common objects present in images are: seashore, lakeside, sandbar, valley, volcano”.

They note their classification of image objects may be incorrect at times, but arrive at the following “impacts” of various objects in photos:

• Strong positive impact: miniskirt, maillot [swimsuit], bikini, cup, brassiere, perfume, revolver
• Medium positive impact: cheetah, giant panda, basketball, llama, plow, ladybug
• Low positive impact: wild boar, solar dish, horsecart, guacamole, catamaran
• Negative impact: spatula, plunger, laptop, golfcart, space heater

via www2014_khosla.pdf.
Other recommendations, from other sources, are to post 1 to 5 photos each day (photo streams show 1 or 5 photos only), and make the last photo uploaded in a group as your best, most eye catching photo (due to how photos are arranged in the photo stream). Tag all photos with search terms and add descriptive text. Cross post photos to groups. And of course, add descriptive texts.
Lisa Bettany made a blog post saying that #1 is to have an out focus background, #2 is to have lens flare, #3 is to have “hot chicks”, #4 is cute animals, #5 is close ups of flowers, and #6 is interesting photos with “interesting and tragically bohemian titles”.