If you convert the images using the image converter to a normal C file, the solution I described here of adding an __attribute__((section(".qspi"))) to the image data array should work.
You would need to look up what the appropriate section to use for your platform is; that does not depend on LVGL. Typically it’s named something along the lines of .ExtFlash but your experience may vary, as this type of thing is essentially up to developer preference.
You will also need to set up your build system so that a separate file to flash to QSPI is produced, and flash that whenever you update the number or content of your images.