You didn't blend it in enough. You can still see the edge of the top layer. Try looking for transparent images, so you don't have use a fading tool. :yes:
Instead of lowering the opacity ( I assume there is such a thing in GIMP ), you might want to play around with the setting or perhaps you could use some gradient maps.
^- I agree, I think in this case, Iron Man shouldn't be transparent... it looks a little off.
Toxic, You mean opacity? Yes there are many options in GIMP for dealing with that.
dotDavid, I second that, and if he can't find any you can always use "color to alpha" as well as many other of the alpha related tools to do that yourself. You just have to consider with, "color to alpha" what color your using as you may effect more of the image than you want to. Gotta love the undo button.
I would also say the right side sticks out a bit, you may want to take the background and mirror one side the other.
You can use the mirror tool or you can copy one side of the background, paste it and do a horizontal reverse and attach it.
Then you can work on Iron Man.
I rather like GIMP and even prefer it to Photoshop but allot of people experience a learning curve with GIMP, you're not alone.