What kit are you using? The on-chip bram is in most cases unsufficient to store image data, but most of the kits available today include some kind of DRAM memory. However, such memory requires an additional controller. If you have the access to the full version of the Xilinx tools, you can find such IP-cores there. If you don't want to talk directly to the memory controller, you could use some kind of softprocessor.

i'm using spartan 3E.