I know SHIFT+i but that deletes all.
EDIT:
Found the answer, sorry
[i] to add new brush image and toggle on brush image edit. It opens a little rectangular selector. More on that one further down!
[Ctrl]+[Shift]+[i] toggles color mode, either using the original colors of the images, tinted by current color, or only use current color, ignoring image colors.
brush image edit ACTIVE:
[Shift]+[i] deletes only the previous image. [don't ask, I wasn't done, yet!]
[Ctrl]+[i] paste currently active brush image. [all just remains of my tests]
[j] toggles perspective transformation on/off, approximating the perspective of the image taken as suggested by your selector rectangle.
brush image edit OFF:
[Shift]+[i] to delete all images. [all still temporary]
[Ctrl]+[i] toggles paint modes. When ON it will use the color and material of the image. When OFF it will flatten the material. Using Average when OFF works like standard Average blending. When ON Average samples the color in the center of the brush and paints the complete image tinted by that color!