Is it possible to layout a grid that has overlapping columns? Without the ability to upload an image, this is quite difficult to explain, but in a nutshell:
Large square image floated left (100% height, 40% width)
Three half sized images horizontally to its right (50% height, 20% width each for a total 60%)
Two half height elements beneath the three half sized (50% height, 30% width each for a total 60%)
Does that make sense?
They also need to be responsive with one of the three half sized ones dropping out on mobile.