Steganography - retreive secret text by comparing original and text containing picture

I have 2 pictures 1st is st_original.png and the other is st_secret_key_container.png . The 2nd picture is the same as the 1st but contains a secret which is the MD5 checksum of some key ill need to decrypt a small text (the background is black in the 2nd one) .

I know this might be unwelcome here but it is a university assignment, i have to retrieve the md5 from the picture and reverse it so i can decrypt with it the text .

i'm not asking to do the work for me,actually im enjoying figuring this out , that's why i didnt upload the pics nor the encrypted text . I tried some things so far ... tried to check for a watermark in the pic with OpenPuff and StegoMagic to no avail .Most of the tools i found searching google are out of date or win 95 or unmaintained since the 90s.

Any idea how should i tackle this ?