Using the data storage type defined on this page for raster images, delegate writing a JPEG file through a pipe using the output_ppm function defined on this other page.

There are various utilities that can be used for this task, for example: cjpeg (package "jpeg-progs" on Linux), ppmtojpeg (package "netpbm" on Linux), convert (from ImageMagick, multi-platform).

convert[image_,out_]:=Module[{process=StartProcess[{
"wolfram","-noinit","-noprompt","-run",
"Export[FromCharacterCode["~~ToString[ToCharacterCode[out]]~~"],ImportString[StringRiffle[Table[InputString[],{4}],FromCharacterCode[10]],FromCharacterCode[{80,80,77}]]]"
}]},
WriteLine[process,image];
WriteLine[process,"Quit[]"];
];