OK, I just spent about 60 minutes typing a fairly comprehensive guide for Steve to do this,assuming I correctly understood what Steve wanted to do, and the Forum promptly went and lost it as I pressed Preview. A timeout loses the article when it goes to ask you to Login once more, even though it allowed you to continue typing which had no hope of being submitted, grrr.
So, a much briefer posting.
Conversion - use Photon application, or functions such as FJPEG in the PHGTK toolkit, both available from
http://www.dilwyn.me.uk/graphics/index.html , to convert the JPEG to a PIC file. PIC files will be in the same screen display mode as the QL display. Photon can convert to QL modes, whereas I *think* (I stand to be corrected, as I haven't checked) that PHGTK only converts to the higher colour modes. You could also do this using sprites, but it's a much more complicated subject to get to grips with.
Display. Once you have the PIC file version of the JPEG, it's easy to display a section of that using the FPIC_LOAD function from PHGTK. It's far easier to do this on a machine with pointer environment (e.g. an SMSQ/E system such as QPC2) than on "vanilla" QL without pointer environment, as the PE does much of the "heavy lifting" for you.
You need to study those extensions in PHGTK (they have so many options to choose from) and make a listing something like this. Be aware that you should convert the JPEG to a PIC file with the same mode number as that which you will be using, e.g. mode 32 for QPC2 high colour, or mode 33 for Q40.
(Note: retyped, unchecked for errors)
REM create a PIC called win1_ql_pic from the JPG file for display in mode 32
error% = FJPEG(win1_photo_jpg,win1_ql_pic,32)
IF error% < 0 THEN PRINT"Oops, conversion failed.":STOP
REM load a section of the PIC into top left of window #1 of size 448x200 pixels
REM see PHGTK manual for meaning of the 8 parameters to FPIC_LOAD
error% = FPIC_LOAD(#1,win1_ql_pic,0,0,0,0,448,200)
DELETE win1_ql_pic : REM tidy up after use
IF error% < 0 THEN PRINT "Oops, problem displaying the PIC"