Well, you can have 1024 x 1024 if you want. Or 1024 x 2.
The error "image 800x600 instead of 2^n" means that the image size is not power of 2 (2^n).
2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192
are all power of 2 (2^n) sizes.
Try making your image 1024 x 512 or similar and see if it works.
Cheers, KTottE