To modify your photos :
1/ Go to the tab " my boats ",
2/ Click on the button "modify the ad", go to the tab " photos " on the left.
You will find here all the photos related to your ad
You can :
change the size/crop by clicking on the small pencil under each picture,
delete an image by clicking on the small bin / trash can under the image
choose your cover photo by clicking on the star under the chosen image.