I think posting stuff like post #203412 should be strongly discouraged in favor of real vector graphics. It is stupid to waste several megabytes of server space on giant rasterization of 10 KB svg.

There is a 750GB drive holding all our images. (As well as a backup drive.) If we run out of space, we'll just install a RAID card and do a RAID5 for drives we connect and serve images that way.

It is certainly nice to know that gelbooru won't run out of space, but the other point still stands. SVG is superior to such bloated PNGs in every aspect, why not gently remind users to upload better files?

In most cases, they do not have the actual vector file. At the time of this post, I have not come across an artist who distributes their vector work without rasterizing it first. They give it to you as PNG or TIFF, but not much more than that. While we could remind them to post that type, which we do not support for certain reasons, the odds of them taking notice of the notice is very slim.

forum #28 is my case and point. I still receive at least 5 reports a day about how much they like a picture.

One thing that could be done about PNG pictures is increasing compression.
There are several tools that allow trying more algorithms to further reduce the size of PNG pictures while still not loosing anything (except weight ^^ ).

Just to take that same post #203412 as a concrete exemple, I used PngOptimizer on it and size went down from 1,356,836 bytes to 1,037,194 (yes, that's almost 1/4 gain oO).
Another program might even do better, as PngOptimizer wasn't designed to do the ultimate possible optimisation.

I'd also advise to always convert BMPs to PNG.

That's actually a pretty good idea. I'll put that suggestion in the tracker.