Nothing on the internet is truly permanent- I expect what they mean is that it's not just a pass-through, but they leave it hosted.

For images meant only for online viewing I resize files at 72 dpi (that's the old standard, the new standard is a bit larger, I think). I usually have them about 10 inches wide.

For viewing an image hardly ever needs to be much more than 200kb- most of the ones I shared over the years were far less than that. Your line drawing dragon I could probably have reduced to 30kb while keeping it the same number of inches visually and not looking much different on screen (it would have been a mess if enlarged and no good for printing, but that's actually a good thing - it makes image theft less profitable.)