I am trying to extract CG files from a visual novel game (Mother Goose no Himitsu no Yakata [JPN]).
Using UMDGen, I found out a file called CG.cpk which I assumed contained the CGs. Then I extract the CG.cpk with quickbms (cpk script), but it gave me a bunch of no extension files which I have no idea what to do.
So now I extract the script.cpk to see what the CG files extension is and I found out from the script that it's in .tga, but when I renamed my no-extension files to .tga, I still can't open it (at least with quicktime picture viewer, it told me that there is an error).
When I tried checking them in trid, it told me it is a GIM file and I don't know how to open it either. The file extension site told me I can open it with ROCeditor, GimpConv or PSP Theme Creator, but I don't think it's related with CG, not to mention, when I tried, none of it worked.

Now, my question is, how can I open those files and convert them to jpg or png?

Thanks in advance.