I am developing a routine to scale some bitmap images to be part of tile notifications for my Window-8 app
The tile images must be <200KB and less than 1024x1024
Considering that tile size either 150x150 for square tiles or 310x150 for wide tiles you should be able to shrink image down to the appropriate size and with jpeg compression you are pretty much guaranteed to be under 200k. Set compression quality around 80. It will give you good compression ratio while keeping decent image quality.