@JadeCandy1 totally got me on the right track, and I suppose the difference I've experienced might be due to differing screen resolutions, or possibly PayPal changed something since this thread started. When I followed their lead my header had awful resolution, and the top was still too cut off. What I did was exactly as @JadeCandy1 JadeCandy1 instructed, except I increased the sizes by 200%, and left the graphic centred, rather than aligning it to the top. Looks perfect now.
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