Usually you upscale the texture - like if you 256*118 sized texture you scale that to 256*128 with GOOD paint prog - it will look streched, but will display correcly if you UV-mapped that using the original image dimensions. 600*450 is problematic because making it power-of-2 would mean that it would become 1024*512 texture... if that's too big, you just have to accept small loss of horizontal accuracy and scale it to 512*512.