9000*4800px is very large, perhaps you could consider making it a little smaller. Looking at your text, I think that it could still be legible, albeit quite small, at 75% or even 50% of the size. At half that size you should be able to get within the size limit with a good quality (like 80% or more using .jpg).
Alternatively, split the image. Since practically no one has a screen large enough to show such resolution, it will be seen only in parts anyway.