is to PICT
is to BMP
'PICT' actually refers to the Picture resource type. Its structure is technically allowed to consist of any number of Quickdraw drawing function calls (meaning it is, in fact, a very simple vector-based format), but in practice PICT resources almost invariably consist of nothing more than a single bitmap image-- meaning PICT comes out to be just a simple way to losslessly store image data.
A "PICT file" is actually just a single PICT resource which has been dumped alone into the data fork of a file. This is the format mac users used whenever they needed to quickly save some image, the same way windows users use BMP, until Mac OS X came along.
As of Mac OS X the TIFF file format has supplanted PICT.