Thank you in advance!
you doing great just some notice :
- you should try to change the window size to check if the page load the correct image or not
- for the picture element you missed :
a) give unit to the width size so it should be 600px not just 600
b) you used 2 max-width i guess it ypo and you ment one of them to be min-width
hope that help and have a nice day