>>That's what I did first, but then that was an early version of this, back in the times when there was never enough disk space. Now I'm reconsidering... though the above link gives some hint to the nature of the JPEG header(s). I was also storing a minimal webpage which would display the picture in a browser object, plus some data (mostly date and time, nothing much). Well, an interesting pastime for, I guess, november, when the weather is... well, you'd call it nice.
>
>You'll find plenty of info here http://www.fileformat.info/index.htm
>eg PNG: http://www.fileformat.info/format/png/corion.htm

OK, here's a version which kind of works for most of the cases (and there are so many of those, it's unbelievable - yet my old faithful ACDSee, from 1998, still reads each one flawlessly):

	Procedure GetJpgSize(tcF)
		Local lcBafer, lnWhere, lnHeight, lnWidth, h
		h=Fopen(tcF,0)
		If Pcount()<2
			o=Createobject("empty")
		Endif
* the FF is first read outside the loop
		lcBafer=Fread(h,8192)

* this is what we are looking for
		Store 0 To lnWidth, lnHeight
		lnWhere=1
		Do While Between(lnWhere, 1, Len(lcBafer)) And Not Between(lnWidth, 257, 8000) And Not Between(lnHeight, 257, 8000);
			and not feof(h)
			lnWhere = At( 0hffC0, lcBafer)
			If lnWhere>0
				lnWhere = lnWhere + 4
				lnHeight = Bitor(Bitlshift(Asc(Subs(lcBafer,lnWhere+1,1)), 8), Asc(Subs(lcBafer,lnWhere+2,1)))
				lnWidth = Bitor(Bitlshift(Asc(Subs(lcBafer,lnWhere+3,1)), 8), Asc(Subs(lcBafer,lnWhere+4,1)))
			Endif
			lcBafer=Substr(lcBafer, lnWhere)
			If Len(lcBafer)<1000 And Not Between(lnWidth, 257, 8000) And Not Between(lnHeight, 257, 8000)
				lcBafer = lcBafer + Fread(h,8192)
			Endif
		Enddo
		Fclose(h)
* some failsafes...
		lnWidth=Evl(lnWidth, 640)
		lnHeight=Evl(lnHeight, 480)
		AddProperty(o, "width", lnWidth)
		AddProperty(o, "height", lnHeight)
		Return o