That’s strange, it never happened to me. I only use one device and I always use the same method to upload an image to the post. The only time when the system changed the image format from PNG to JPEG was when I uploaded very big images.
A cache does probably exist, but I still find it strange that one of the cached versions of the images is a JPEG.
Some CDNs can be configured to automatically transform images, but I don’t think that this is what’s happening here because the CND used for hosting the images shouldn’t have that feature.
That kind of interpolation would lead to significantly smaller sizes but a slightly better compressible image might be achieved also using resampling algorithms alternative to bicubic.
In this specific case the final visual result would be pretty much identical regardless of the used algorithm, because the differences in the interpolated pixels are too subtle to be recognized by the eye. The only important thing that changes is the size of the image.
Just as a proof of concept (I hope you don’t mind talking about this absolutely minuscule aspects, I just like experimenting with graphic algorithms and compression ) I have taken your original “cemetery fullsize (v2).png” and I have proportionally downscaled it (from w2358 to w1920) using different resampling algorithms and switching on the maximum PNG compression level (value “9” on my software):
|Compression 9||Compression 9 + lossless optimization with special software|
|B-spline||174 KB||168 KB|
|Hermite||191 KB||184 KB|
|Lanczos||227 KB||219 KB|
|Mitchell||193 KB||187 KB|
|Triangle||186 KB||180 KB|
…which make us conclude two things:
The choice of the resampling algorithm can significantly change the size of the file (the difference between B-Spline and Lanczos is +30% in size).
Regardless of the resampling algorithm, further lossless compression is possible with special software.
Size optimization is a concern for every user that has to download files. In this specific case we are talking about a website with a small quantity of traffic, but as a general rule losslessly compressing images (even further) is an activity beneficial to everyone.
“Similar” does not imply the best kind of optimization. Actually, if Discourse observes that they cannot create a smaller file from the manually optimized one that I uploaded, the CMS should just keep my file.
The only way to create an even smaller file would be to send a lossless WebP version of the image to those clients that support this format. But it doesn’t seem to me that this mechanism is in place, on this website.