The main reason for not including full size pictures is, as canyonag77 points out, that the owners of the 'Bolt prefer that you not.
You are mistaken, however, when you say that simply hosting pictures elsewhere means there is no impact on the 'Bolt's bandwidth. There is no impact on the 'Bolt's storage, but there is an impact on bandwidth.
Most web hosting companies give you a certain amount of storage space (the room on the server's hard disk that can be used to store data) AND a certain bandwidth limit (the number of bits and bytes that can travel through the 'pipes' each month). When you post large pictures which are stored on your server in the threads, there is no impact on the 'Bolt's total storage space. However, when anyone goes to the thread to view your post, the 'Bolt's server requests the bytes that represent your picture from your server. Bandwidth is used to bring the picture to the 'Bolt. Then the 'Bolt's server has to send that image to the viewer's computer, using more bandwidth.
Bottom line, however, is simply that the owners of the 'Bolt ask that we not include pictures in our posts.
HTH
