The problem with JPGs as opposed to BMPs is that JPGs can vary in DPI ( dots per inch)
Lets say you have a bmp that is 100 x 100 and you have a jpg100 x100 that has 75 dPI and another JPG 100 x100 150 DPI, even though they all have the same picture size, they will all display at different sizes in Mach. This is because Mach, unlike most picture displayers reads, the JPGS not by the picture display size, but by the DPI. I am not sure If Art was aware of the differences between BMPs and JPGs when he made mach able to display JPGs.
Having this display issue causes many design problems.