Hello,
1) If you want to change the first menu level go to Appearance > Menus > edit the main menu. To change the mega menu watch the video tutorials how to do it using Static Blocks https://xstore.helpscoutdocs.com/article/27-mega-menu and edit it according to your needs.
2) I don’t see the problem with images on your site https://gyazo.com/1c1d633f50625b3386e943b3c101123f Could you describe in more details or provide screenshot?
Regards