@Alex- I can't really give out much details since this problem never happened to me before, though I will answer the four questions you asked.
-It wasn't on .png, yet it still wouldn't work when I changed it.
-It wasn't 513x513, yet it still wouldn't work when I changed it.
-It was in RBG, yet still didn't work.
-If you are referring to the actual files, they are saved under my_presets in the correct folder; if you are referring to the .fhm file, it is saved under the "presets" folder.