Adding images to columns

Hello,
This is a bit of a tricky one to explain.
I have used the 4 width column insert into a page. I have inserted an image into each column. The images in column 1 and 4 are longer than the images in 2 and 3, thus there is a blank gap in the lower middle section. I want to add some text or another image into this gap, but cant get access to it. Does anyone have any ideas how I can get around this?