This appears to be an issue with the 3.5E-Monsters module. Some of the monsters in that module (as you found) do not have images so it is causing an issue the first time that creature is loaded. This is without any other modules loaded. This error also appears if loading a 3.5 campaign once again without any other modules loaded but base. You don't really need to load 3.5 modules as either PF core and bestiary or PFRPG Essentials (its 4 modules) have the content need to run PF1.