Nice. At least your better then me at worldpainter :p

Hey its been a long time since I used MC edit or played minecraft in general but this is what I remember:

How to make an island:
Make a lot of jungletrees next to each other so you get one huge tree 
Go to MC Edit and filter the tree to "only stone" 
copy it and put it in the air
Floating island ;) 
(Not sure if this (still) works :p)

Hey welkome to planet minecraft 
I think your island would look a lot better if you make more round and without straight lines. If you want to make smal floating islands i would suggest to use the brush tool ofworldedit or voxelsniper to make the shape. Beleve me it will look more natural 
If you want to make bigger islands i would suggest to use world painter with custom cave layer as method. If you do this you should use some custom brusches to rais mountains and hills 
If you have some questions feel free to send me a personal message or add me in skype ( name : jannekloeffler )