netpbm might do what you wish. there are hundreds of programs, most suitable for streaming together.

$ djpeg foo.jpg | pnmscale -xy 100 100 | cjpeg -smoo 10 -qual 50 >smal
+l.jpg


SEE ALSO
     anytopnm(1),  asciitopgm(1),  atktopbm(1),   bioradtopgm(1),
     bmptoppm(1),   brushtopbm(1),  cmuwmtopbm(1),  fitstopnm(1),
     fstopgm(1),  g3topbm(1),  gemtopbm(1),  giftopnm(1),  gould-
     toppm(1),    hipstopgm(1),    hpcdtoppm(1),    icontopbm(1),
     ilbmtoppm(1),  imgtoppm(1),   lispmtopgm(1),   macptopbm(1),
     mgrtopbm(1),     mtvtoppm(1),    pbmclean(1),    pbmlife(1),
     pbmmake(1),    pbmmask(1),    pbmpscale(1),    pbmreduce(1),
     pbmtext(1),    pbmto10x(1),   pbmto4425(1),   pbmtoascii(1),
     pbmtoatk(1),  pbmtobbnbg(1),  pbmtocmuwm(1),   pbmtoepsi(1),
     pbmtoepson(1),  pbmtog3(1), pbmtogem(1), pbmtogo(1), pbmtoi-
     con(1), pbmtolj(1), pbmtoln03(1), pbmtolps(1), pbmtomacp(1),
     pbmtomgr(1),  pbmtopgm(1),  pbmtopi3(1),  pbmtopk(1), pbmto-
     plot(1),    pbmtoptx(1),     pbmtox10bm(1),     pbmtoxbm(1),
     pbmtoybm(1),  pbmtozinc(1), pbmupc(1), pcxtoppm(1), pgmbent-
     ley(1), pgmcrater(1), pgmedge(1), pgmenhance(1), pgmhist(1),
     pgmkernel(1),     pgmnoise(1),     pgmnorm(1),    pgmoil(1),
     pgmramp(1),   pgmtexture(1),   pgmtofs(1),    pgmtolispm(1),
     pgmtopbm(1),    pgmtoppm(1),    pi1toppm(1),    pi3topbm(1),
     picttoppm(1), pjtoppm(1),  pktopbm(1),  pnmalias(1),  pnmar-
     ith(1),  pnmcat(1),  pnmcomp(1),  pnmconvol(1),  pnmcrop(1),
     pnmcut(1),    pnmdepth(1),    pnmenlarge(1),     pnmfile(1),
     pnmflip(1),  pnmgamma(1), pnmhistmap(1), pnmindex(1), pnmin-
     vert(1), pnmmargin(1), pnmnlfilt(1), pnmnoraw(1), pnmpad(1),
     pnmpaste(1),    pnmrotate(1),    pnmscale(1),   pnmshear(1),
     pnmsmooth(1),   pnmtile(1),   pnmtoddif(1),    pnmtofits(1),
     pnmtops(1),  pnmtorast(1),  pnmtosgi(1), pnmtosir(1), pnmto-
     tiff(1),     pnmtoxwd(1),     ppm3d(1),      ppmbrighten(1),
     ppmchange(1),     ppmdim(1),    ppmdist(1),    ppmdither(1),
     ppmflash(1), ppmforge(1), ppmhist(1), ppmmake(1), ppmmix(1),
     ppmnorm(1),  ppmntsc(1),  ppmpat(1),  ppmquant(1),  ppmquan-
     tall(1),     ppmqvga(1),     ppmrelief(1),      ppmshift(1),
     ppmspread(1),    ppmtoacad(1),   ppmtobmp(1),   ppmtogif(1),
     ppmtoicr(1),   ppmtoilbm(1),   ppmtomap(1),   ppmtomitsu(1),
     ppmtopcx(1),    ppmtopgm(1),    ppmtopi1(1),   ppmtopict(1),
     ppmtopj(1), ppmtopjxl(1), ppmtopuzz(1), ppmtorgb3(1), ppmto-
     sixel(1),     ppmtotga(1),     ppmtouil(1),     ppmtoxpm(1),
     ppmtoyuv(1),  ppmtoyuvsplit(1),  psidtopgm(1),   pstopnm(1),
     qrttoppm(1),    rasttopnm(1),    rawtopgm(1),   rawtoppm(1),
     rgb3toppm(1),   sgitopnm(1),    sirtopnm(1),    sldtoppm(1),
     spctoppm(1),    spottopgm(1),    sputoppm(1),   tgatoppm(1),
     tifftopnm(1), xbmtopbm(1), ximtoppm(1), xpmtoppm(1), xvmini-
     toppm(1),    xwdtopnm(1),   ybmtopbm(1),   yuvsplittoppm(1),
     yuvtoppm(1), zeisstopnm(1)
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