Well, to save storage space for Scrubs I often convert my .png renders with MS Paint to .jpeg before uploading. I can't notice much (if any) difference between the two .png and .jpeg. I think the problem is how it's compressed to a smaller res, on here at least. I haven't noticed any issues with flickr before.

To convert with Paint, right click the file and select Open With --> MS Paint/Paint. Then click the blue tab in the top left, and hit Save As --> JPEG.
Pretty sure it would only work on Windows though.

A plain dark background that yields a gradient is very much vulnerable to loss of quality, this is noticeable if you convert a .png into a .jpeg yourself, or after Flickr has converted it. I suppose Mecabricks does the same, but does it store the originals? Hence a couple more questions:

4. Will uploading .jpegs to Mecabricks save Scrubs any space? 😃

5. Are there other background options less susceptible to quality loss? Like changing the material, or maybe rendering without a background per se and adding one later in Photoshop?

Inputs on any of the 5 questions are much appreciated!

Yes, the originals are stored. If you click the download button on the renderings tab, the image you download will be the one originally uploaded.

1. I would say there would be benefits to 16 bit if you're going to work on it in post, or if you want a less lossy file for yourself.

2. I usually use OpenEXR for my original output file, as it contains multiple exposures to tinker with in post, and can be lossless if you so wish.

3. Yes, it will save space. For example, a .png I have is 13.7MB, while its .jpeg counterpart is only 2.98 MB. Feel free to try it out yourself. 😉

4. There would be, but I can't say what for sure. Something that has more colour/texture variation would likely have less banding than a blank plane would.

Yes saving in jpg will save a lot of space and if done properly you will often not see the difference for most cases. I will struggle with storage in a not too distant future and I will have to find some solutions.

Thanks very much for your replies, guys! This clarifies quite a few things.

I did some tests by picking out "the worst offender" with most visible banding. Well, the difference between 8 and 16 bit PNG is surprisingly slight, hardly visible at all to my eyes. Whereas JPG is universally bad regardless of the source.

Seems the key is to liven up the background using texture, lights, different shaders, etc. On a different note, I will also play around with OpenEXR in post-editing, thanks for the tip.

Thanks again and if anyone else can contribute then please do so!

You're welcome, glad to help!

I recommend editing your OpenEXR in your Blender file that you rendered it in to get the brightness right. Other programs will change how they interpret it, and it can totally change how the original picture you had looked. Essentially, use it as a safety in case Blender crashes. 😉