kaiden Emerald

hi. ive been trying to upload a couple of custom textures that i made to see if i could upload my own textures. i run a mac, and i use gimp. my problem is, when try to upload it, it says that it wont work. ive made the files .tga files, but one thing im not sure of is how to check how many bits it is. im using these textures for building stuff, by the way. im not creating clothing or tattoos. so, any help would be appreciated. thanks.