script for extract and replace JPEG images in samsung SBL.img (bootloader.img). Tested on maguro (i9250).

extract_jpeg.py

#!/usr/bin/env python3

#            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
#                    Version 2, December 2004
#
# Copyright (C) 2014 Souren Araya
#
# Everyone is permitted to copy and distribute verbatim or modified
# copies of this license document, and changing it is allowed as long
# as the name is changed.
#
#            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
#   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
#
#  0. You just DO WHAT THE FUCK YOU WANT TO.


# THIS PROGRAM CAN COMPLETELY BREAK YOUR PHONE! YOU HAVE BEEN WARNED.

from os.path import isfile
from io import BytesIO

soi = b'\xff\xd8\xff\xe0'
eoi = b'\xff\xd9'

def scan(source, verbose=False):
    f = open(source, 'rb')
    found = []
    last_offset = 0
    while True:
        f.seek(last_offset)
        start_offset = f.read().find(soi)
        if start_offset != -1:
            f.seek(start_offset+last_offset)
            length = f.read().find(eoi)+2                   # FFD9 bytes
            if verbose:
                print('JPEG found at offset %i, length %i' % (last_offset+start_offset, length))
            found.append({'offset': last_offset+start_offset, 'length': length})
            last_offset = last_offset+start_offset+length
        else:
            break
    return found

def extract(source, offset, length, extension='jpg'):
    f = open(source, 'rb')
    output_name = '%i:%i.%s' % (offset, length, extension)
    outfile = open(output_name, 'wb')
    f.seek(offset)
    content = f.read(length)
    if content:  
        outfile.write(content)
        outfile.flush()
        outfile.close()
        print('File %s written.' % output_name)
        return True
    else:
        print('Invalid offset %s. Source file length is' % (offset, len(f.read)))
        return False

def extract_everything(infile):
    if not isfile(infile):
        print('Source file does not exist\nExiting.')
        return False
    images = scan(infile)
    print('Found %i images.' % len(images))
    if len(images) > 0:
        print('Extracting...')
        for i in images:
            extract(infile, i['offset'], i['length'])
        return True
    else:
        print('Exiting')



def replace(output, source, offset, length, verbose=True):
    if not (isfile(output) and isfile(source)):
        print('Source file or output file does not exist\nExiting.')
        return False
    
    outfile = open(output, 'r+b')
    sourcefile = open(source, 'rb')
    sourceobj = sourcefile.read()
    if not len(sourceobj) <= length:
        print('Source file does not fit. Correct size is %i bytes or less.\nExiting.' % length)
        return False
    outfile.seek(offset)
    tmpfile = BytesIO(b'\x00' * length)
    outfile.seek(offset)
    outfile.write(tmpfile.read())
    outfile.seek(offset)
    outfile.write(sourceobj)
    outfile.flush()
    outfile.close()
    if verbose:
        print('Replaced succesfully')
    return True


if __name__ == "__main__":
    import argparse
    from os.path import basename
    ap = argparse.ArgumentParser()
    ap.add_argument('-e', nargs=4,
        help='extract fragment of SOURCE by OFFSET with LENGTH and EXTENSION of output file\n %s -e sbl.img 1118428 3305 jpg' % basename(__file__),
        metavar=('SOURCE', 'OFFSET', 'LENGTH', 'EXTENSION'))
    ap.add_argument('-s', nargs=1,
        help='find JPEGs in SOURCE, print offsets to console\n %s -s sbl.img' % basename(__file__),
        metavar=('SOURCE'))
    ap.add_argument('-a', nargs=1, help='extract all JPEGs from SOURCE\n %s -a sbl.img' % basename(__file__),
        metavar=('SOURCE'))
    ap.add_argument('-r', nargs=4,
        help='replace fragment of OUTPUT with SOURCE by OFFSET with LENGTH\n %s -r sbl.img 1118428:3305.jpg 1118428 3305' % basename(__file__),
        metavar=('OUTPUT', 'SOURCE', 'OFFSET', 'LENGTH'))

    opts = ap.parse_args()
    if not any([opts.e, opts.s, opts.a, opts.r]):
        ap.print_usage()
        quit()


    if opts.e:
        extract(opts.e[0], int(opts.e[1]), int(opts.e[2]), opts.e[3])
    if opts.s:
        scan(opts.s[0], True)
    if opts.a:
        extract_everything(opts.a[0])
    if opts.r:
        replace(opts.r[0], opts.r[1], int(opts.r[2]), int(opts.r[3]), True)

dump your SBL partition:
dd if=/dev/disk/by-partlabel/sbl of=/home/nemo/sbl.raw
modify, than writ it back:
dd if=/home/nemo/sbl.raw of=/dev/disk/by-partlabel/sbl

easy way to get suitable image:
convert -define jpeg:extent=LENGTH input.png output.jpg