You mean truecolor header , but this is lvgl missinformation. Check real bytes in array… And read Images — LVGL documentation
Online converter is in docu and you can use it for create c files , that you manualy swap or add into squareline generated project. For example i use RAW for png store in code and decode on show or CF_INDEXED or CF_ALPHA for save space on simple images.
And ofcourse Squareline project default depth is 16 bit.