A quick comparison how fast images load on the iPhone using a simple UIImage imageWithContentsOfFile:. Each image is 8-Bit RGB without Alpha:
Filename
Bytes
msec
img-10000×50.png
224193
283
img-50×10000.png
200397
643
img-900×600.png
879483
623
img-30000×100.png
727709
1222
img-150×20000.png
911923
2218
Looks like the horizontal strip is the winner when it comes to loading speed.