Most people recommend making a separate partition for storing data on a external drive that has a Time Machine backup on it; however, I've stored data without a new partition and it didn't seem to affect Time Machine.
As Eric Root pointed out there is no requirement to have separate partitions. The reason most people do it is because Time Machine will take as much space as it can. So if you have your iPhoto library and your time machine backup on the same partition you may find yourself running out of room for new pictures before you expect as Time Machine keeps gobbling up hard drive space. HTH.