I have amended my website layout by adding one photo on the page in the ipad mode (753-1000px) and phone mode (320-752px), however when I switch my layout to Mac mode (1001px or above), my newly added photo was disappeared. May I know is that a bug and what follow-up actions could be done in this case? Thank you.
Most probable reason is your image is hidden on desktop layout. When you add an element in smaller breakpoint, it is often hidden on larger breakpoints(they are added on lower breakpoints though).
Select your image from layers on tablet editor, go to desktop editor and make it visible(right click and unhide).
Another reason can be the image is hidden behind other layers, to fix that right click on image, go arrange, and bring it forward.
I think 1st reason is most probable.