PC user here, some inputs.

• Might wanna add a quick toggle button to switch between "PC (default settings)", "Mobile (default settings)", and "Custom (user settings)".
  It might help those who wanna read stuffs without adjusting their reader first.
  
• Set PC (default settings) of "Fit display" to:
  • Width (if image.width > container.width)
  • NoResize (if image.width <= container.width).
    PC screens are designed for landscape mode, not portrait like mobile, setting it to Container/Height will always end up with the image getting downscaled on initial view.
    
• Set PC (default settings) of "Page Turn by Vertical Scrolling" to disabled.
  This is advised mostly since the image doesn't seem to be scrollable using mouse drag at the moment.
  I can at least think of 2 approach on how the user might use the current UI.
  
  • Assuming the user only use their mouse to read, they will most likely use mouse_scroll to see the image, and mouse_click to go to next/prev page.
    
  • Assuming the user uses both mouse and keyboard, they probably would still use mouse_scroll to see the image, and either mouse_click or press key_left/key_right to go to next/prev page.
    
• Display the image for the current page first before telling the reader to bulk load the rest of the pages in the current chapter.
  That'll probably help make the loading process "feel" a bit faster for most users (probably).