Try saving the name without capitals and spaces... If that doesn't work, try re-making the heightmap. Make sure it is in terrains, give it a weird name, this sometimes happens with common names. Grayscale, and all on the background or bottom layer. Uhm... Trying to think of anything else... 513 by 513, or even 512 by 512, do you have a lot of lag? Try closing all other programs apart from FH when you enter it (Especially internet/google and skype) Send someone else the .png and see if it crashes theirs.
Hope you fix it.