When setting up the Render settings you should be able to see what the final image format will be by checking the filename, is it .Jpg, .Bmp or .Png, change it to what you want. If you want transparent PNG make sure Transpaent Png is enabled in the Options list. I have a measly 3gig of ram but have rendered 25 and 30 mpx Pngs before no problem, there is however a warning that pops up in the image output stage that says "when rendering Higher Resolution Images it is better to use BMP format to avoid Memory problems" I ignored it and the Png was fine.

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