To clearly see the source of posterization, I loaded the image in PS, added a curve adjustment layer to it and moved the top right corner of the line to the left, until Input 25, Output 255 or so.
What I see is
1. the posterisation (at least that, what is visible on my monitor) comes from the shadow on the cloth (you should iron the cloth to eliminate the strong lines of folding :-).
2. there are quite large chunks of identical pixels. 90 kilobytes for a 600x900 JPEG image is not much, i.e. the degree of lossiness is high (compared to this background). Higher quality setting makes the chunks smaller, i.e. the transitions appear smoother.
If you created this image from a larger, higher quality JPEG, take a look at that one the same way. Was that quality high enough? If you have the image in TIFF (i.e. in raw converted in TIFF), then create the Web version from that one, not from an intermediate JPEG.
If it is raw based, then perhaps increasing the black level would eliminate this issue all together by making the background pure black.