Convert the .iwi that you found into .dds then into a image format which you can edit. There are tutorials for this.
So .iwi > .dds. > some image format idk which (jpeg, png)
I mean, I did all that. Here's my steps in depth.
> I took extracted copies of the files I needed.
> I put them in the IWI x DDS program in the TOOLS.rar
> I drew all over the .DDS without making it another image format
> I finished, converted it back to IWI, and ran it and the original one in the CRC32 Fixer
> I checked they had the same file size (they did)
> I put them back in the right iw_##
> I got an error