Trying quality reduction, via:

$ jpegoptim --strip-all -t -m75 *.jpg
CC_BY_NC-flickr_ericparker.jpg 140x140 24bit JFIF  [OK] 16315 --> 5184 bytes (68.23%), optimized.
CC_BY_NC_SA-flickr_manachar.jpg 140x140 24bit JFIF  [OK] 11913 --> 3958 bytes (66.78%), optimized.
CC_BY_NC_SA-flickr_seandreilinger.jpg 140x140 24bit JFIF  [OK] 16235 --> 4193 bytes (74.17%), optimized.
CC_BY_NC_SA-flickr_umhealthsystem.jpg 140x140 24bit JFIF  [OK] 15378 --> 4633 bytes (69.87%), optimized.
Average compression (4 files): 69.76% (41k)

I may revert this, we'll see.