The preview is what you see when you host the game imo you don't need a large pic that takes space. Most previews I've seen are 128x128. Anyway, here is a shrinked version 128x128, and 256x256. Yours was 252x252 and textures in war3 need to be a power of 2. 2^8= 256
The name in import manager is: war3mapPreview.tga, remove the war3mapimported\ part.