The CMOS imager does a huge amount of work and gets very hot, the IBIS makes it much harder to cool too, so this is a big issue that they need to handle. Typical CMOS imagers have light gathering sites, amplifiers for each pixel (hopefully), A-D convertors (again hopefully each pixel), memory for each pixel (hopefully), and of course a data bus controller to get the data out of the imager. Lots of stuff generating heat in that single chip. So it's not just the processor that gets hot and needs to be cooled.