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

I have 2 pictures: st_original.png and st_secret_key_container.png .

The second picture is the same as the first 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 second 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 I'm enjoying figuring this out , that's why I didnt upload the pics nor the encrypted text . I tried some things so far like checking for a watermark in the picture with OpenPuff and StegoMagic to no avail .Most of the tools I found searching google are out of date or win 95 or unmaintained since the 90s.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 ?

EDIT : a text is definatly hidden in the picture its no question of "if" , its clearly stated in the assignment that its a text hidden via LSB (least significant bit) stenography.

UPDATE: ok i spent the whole night till 6 am trying to figure out this thing , i understand now how LSB steganography works , but the problem is the PNG format and the black background .If its only hiding an MD5 (32 chars) then not much should really change . But the black background makes it hard cuz alot changed.