When I make a banner, I look for an image that (at full resolution) has a nice pretty group of blooms taking up the entire image. Like this one for example:
Then I open it full resolution (I also use the gimp), zoom out so I can see it all, and then I use the razor tool to cut out from edge to edge a nice long thin selection that looks good. That'll usually give me a 500 high by about 5000 wide image. Then I rescale it down to 150 height and I end up with about 1500 pixels wide. I hope that info helps!
