In Get started with Google Earth Pro - Google Earth Help they say “images greater than 2000x2000 pixels can reduce the performance of Google Earth”.
I have tried much larger overlays (10000 x 7000) and they do not “reduce the performance” but they appear all blurry and unreadable. It seems GE just reduces the definition to crap.
I am trying to overlay nautical charts and, frankly, dividing each chart into 20 sections would be a useless PITA. I want the whole chart in one overlay and I want it readable. A 4 Mpix image overlay might be useful for some things but it is useless for overlaying a nautical chart with depths etc unless the area is smaller than my living room.
Large images are overlayed while being readable like the maps at http://www.davidrumsey.com/ but I notice those work by the same technology as the rest of GE by which I mean they stream down and gradually increase detail in the section you are displaying. I do not know how they do that.
A file overlay as done by a user overlays the entire image in one go and with whatever level of detail it will have.
I can use the same image that GE cannot use, I use it with Trackmaker and it works OK although a bit slowly in my 700 Mhz laptop.
What can I do with Google Earth?
By the way, I am downloading the low quality charts from here. If anyone knows where I can get the charts for free with better quality I would appreciate it. Of course, I could also scan my paper charts but I figure they must be available for download somewhere. The basic data is given out for free by the US government but the resellers charge for their work integrating it into their products. There must be a source where one can get the charts for free.