Datatypes are used for loading the backdrops in Workbench 3.0 and on. So, if you don't have any additional datatypes to the original ones you can only use IFF images, but virtually any image can be converted to IFF using most paint packages, such as Personal Paint, Perfect Paint, ImageFX etc.
When you do the conversion, remember to convert down to the number of colours your Workbench is displayed in. The datatypes will do this for you anyway, but a paint package will usually do a better job.
