Odds are the author is mistaken. I'm fairly certain datatypes.library requires some v39 features not available in 2.x. Otherwise I expect everyone with 3.x disks would have copied the datatypes over to their 2.x systems by now.
Maybe datatypes v45 is different. I don't know.
Try looking at the traditional big three of Amiga image processing:
ImageFX, ADPro, and ImageMaster. I think ImageMaster is freeware now, and ADPro turns up fairly regularly on eBay. I think there's also a demo of Photogenics on Aminet.