The first time I started seeing expanded photos in my twitter feed was early July of this year. My first reaction was that one of my chrome extensions was the cause of this happening. So I ran a couple of experiments to test if this tactic was worth pursuing. The metrics that I saw as a result were pretty powerful. As a result I highly recommend adding photos.
Adding a photo expands your tweet to the size of 3x the vertical height of a normal tweet. If a follower Retweets your photo, you now have that space on their feed as well. For the best result use a call to action in the viewable window area.
NOTE: In order to take advantage of this the photo must be uploaded through Twitter.
The dimensions I used:Original Image Size -- 868px w x868px hExpanded Image -- 438px w x 438px hViewable Window Area -- 438px w x 220px h Free PSD Template using these dimensions.
Here are some stats:Tweets with photos are 94% more likely to be RetweetedTweets with Instagram links were 42% less likely to be Retweeted.Tweets with images receive 18% more clicks than those withoutTweets with images receive 89% more FavoritesTweets with images receive 150% more Retweets
Exhibit A: https://twitter.com/GrowthHack/status/405808629527216128 (RT and FVs)