@kaia
What I strive for is below 1 Mb for images — but this doesn't mean that it should be achieved at any cost, avoiding WebPee is still mandatory. PNGs should be used for screenshots if they mostly have text in them and little to no images or JPEG if they have images, but quality should be high enough so text remains readable and there are no visible compression artifacts.
Songs should be in the 5 Mb ballpark, MP3 or AAC for better compatibility.
Videos shouldn't be posted at all in any forms other than links, but having vids below 10 Mb is acceptable I suppose.