Aw, my method's definitely not needed if you know how to save as csv, but I already admitted that I don't know how. (And I still can't do it with your instructions, it seems to save another copy in txt despite selecting "all"; feel free to say it's because I'm an idiot and so on).This is completely untrue you can and should leave both the csv and your custom .pngs in the custom folder and all you really need to do to to make the file a csv is to go "save as" and just name the file custom.csv while changing the filetype to "all".
I also believe you will have a basis for calling my suggestions in general completely untrue and I won't challenge that statement.
I just want to say that I tried all those steps and it worked for me and I have completed entire playthroughs with custom characters by applying the method. There are very likely better ways, but I did not know if anyone else would answer that question.
Last of all, it's good that you offered a faster, more efficient answer to the question.
I'm glad to hear you got the issue sorted out.Thanks! It really helped me clear it up. First times are always a mess!