I heard rumors that Sansar might be (soon) adjusting the rules for the main pictures in the store. I assume because pictures of the clothing designs made outside of Sansar (like Marvelous Designer for instance) look different. This is because the fabric settings in MD are not (yet) compatible with Sansar.
But making pictures in Sansar of your designs (especially on your avatar) is not that easy. The ones among us who know Second Life, know it is way easier there (posestands, static poses and the way the camera works there).
In Sansar you have to work without those tools. So I suggest the following:
The easiest is for the lindens to adjust the "style my avatar" option. There you can rotate your avatar 360 degrees. So if the Lindens build in an option to take pictures there (in 1:1 format with a minimum of 600 x 600 pixels) and change the background to a solid color (of which we can change the color), it should work like a charm. This way all pictures (no matter from which merchant) have the same lighting and shadow casting. And it also makes the most sense to make pictures there, because you can easily and fast change outfit and continue making pictures.