If anybody knows how I can make my map have a Mental Omega style rendered preview, I'd very much appreciate if a step-by-step tutorial was made. I tried CNC Maps renderer, and also tried converting a rendered picture to preview format with XCC and manually pasting it in the map file, but the previews are either huge (take about 3-5 times the disk space official maps do) or have very poor quality or both.
Official maps use separate images (same location and name as map but with PNG extension) for the preview in the client. These are the full-sized map renders that have been resized to 15% of their original size and indexed to a 256-color palette to save on file sizes. Maps themselves do not have custom embedded previews, using ones generated by map editor to my knowledge.
Now the client would be able to load the separate previews for custom maps as well, if it wasn't for a small bug that causes it to try and load them from an incorrect path. Although it probably wouldn't work with the map transfer feature, assuming that even works at all for MO or in general. So as it stands, your best bet is probably making a full size render, resizing it to 15% and using that XCC method to embed it in the map itself. That is guaranteed to increase the map size, though, and I am unsure if it will produce results that are 1:1 with the official map previews.