Originally Posted by
Raptress
/aghconvert requires that you put the file extension of your file. If you are trying to convert a .tga, you need to add ".tga" to the end of your filename when using the command.
If your file is a targa and called "dragon_head_mask01_skull" then the command to use in Istaria woud be: /aghConvert dragon_head_mask01_skull.tga
As Racktor says, this does not replace the original file and instead creates a new version of it in agh form, so you will have both the .tga and the .agh in the end.