'File too big to replace' error

[komi]

I'm trying to install some custom skins with IMG Tool into GTA3.img but the program keeps telling me the file is "too big to replace". I tried deleting the original .txd files and adding the new ones, but all that happens is they end up at the bottom of the GTA3.img list and when I load the game the character models are solid white (aka no texture).

I'm on the Mac version by the way. Anyone know how to solve this?