First off, before you start implementing JPEG decoder on '51, take into consideration that this is quite a heavy computationally task. It may last some reasonable time for something like 128x160 phone display with a clone like 33MHZ/4 clocked, but if you plan on something like 800x600 VGA or using "vanilla" 12MHZ/12 '52, forget doing it in software.

To give you the idea: Amiga computers were quite great for their times and overperformed PCs in many domains. But while JPEG was a standard format for the WWW web for PCs already, I was watching some of these downloaded from the net on my A600.
CPU: 7MHZ Motorola 68000, a 16bit processor with way more commands than '51, and more useful at heavy maths. 2MB graphics memory, hard drive. Displaying a JPEG of something like 800x600 took about 15 minutes. Same thing on A1200 with twice the CPU speed, "fast" memory and FPU took about 30 seconds.

Note decoding JPEG is a lot of floating point maths including trigonometry, something for which '51 is a very bad choice.