There should be a .cfg file in FeralHeart > media > terrains > textures called 'defaultTex'. Write the exact name of your custom texture in that list, including the type of file it is, for example, .jpg. Save it, restart the game and it should be listed.
I don't think the size of the texture matters. Maybe save the image as a jpeg if it isn't already; all the other textures are in jpg format so maybe they'll only work like that.
Edit:
I think there'll also be a Text Document in that same folder, also called defaultTex. Add the full name of your custom texture file into that list as well. Preferably in the same place as you did on the other list - if you added it to the bottom of the list in the other file, add it to bottom aswell in this file. I think that should make the name come up in the map maker. :)