Author: Craig R Morton
MDL-60309 theme_boost: Add background image filepicker to Boost
http://git.moodle.org/gw?p=moodle.git;a=commit;h=6961f78dcae1c0e217e6e0817813570f15af3d54
http://github.com/moodle/moodle/commit/6961f78dcae1c0e217e6e0817813570f15af3d54
+ 3.4 en [backgroundimage,theme_boost]
+ 3.4 en [backgroundimage_desc,theme_boost]