replace .png imgs with smaller .jpgs

- using _sm extension so we know they’re small
- we don’t use the .png files anymore, so they can go
- after running `homebrew install imagemagick`, this was helpful (note, it works on mac): 

  for i in *.png; do N=`basename $i .png`; convert ${N}.png ${N}_sm.jpg; done

- the .jpg files are about 1/3 - 1/4 the size of the png images, but that is expected (see Figure 12 here: http://www.ou.edu/class/digitalmedia/articles/CompressionMethods_Gif_Jpeg_PNG.html).