You can try to search forum, this was discussed soooo many times before.
I think you have to use squared image 1024x1024 with just added solid color borders. Then you have to draw the image with function draw2DImage with core::rect<s32>(0,0,800,600) (see API)
Your problem - in image size - it MUST BE POWER OF 2!!! (and of course your main problem - inability to search in the forum but ask, ask and ask again...)