Good evening! If you don't find the '.png' type of file when saving it, why don't you just change it's name to end in '.png'? It might work.
And do open the map in FH, you need to go to the map maker. On the open tabs, you'd click on 'worlds'. The first thing listed on there would be 'Terrain Height Map'. Under that, there's s space for you to type. Erase whatever is typed there and type the name of your map, for example 'HeightMap1.jpg'. If you didn't understand very well with my explanation, here's a useful tutorial: http://feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=18937.0%20newwindow (http://feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=18937.0%20newwindow)
Good luck!
Oh, I see. Well, if you need any help, just please, let me know! And yes, I do. Click on 'Profile' behind the 'Hello Alastar' on the begining of the page. Then click on 'forum profile information' under 'modify profile'. There, you can change your avatar and signature, that's the image/text located under what you type. Good luck!