Might depend on the images themselves - some would look just fine as more efficient formats than jpg. 5000x5000x3 monochrome or grayscale could succeed as gif...

Do you care much about the in-between spaces -- or another way to ask - are these three *small* continents such that the world at the same scale would be 30,000 x 60,000 pixels? Or do these 5k sq views add up to the whole world already? A collection of zoomed-in continent maps and an overall locator map at vastly smaller scale makes sense if the oceans are featureless voids. If they have important islands, trade routes, currents, and suchlike, 'tis a different matter.

What's the mapping for?