I assume you mean digital pictures (photos). I ask because you mention Quicktime and that is for video clips.
We there is no real Windows default for pictures, although most people use JPG.
BMP is Bitmap which is not that great for storing pictures (files are large).
Most people store pictures as JPEG (jpg).
You can also use GIF but that is not really good for digital pictures, more for icons and cartoon images.
There are various tools around to convert images from one type to another, I think IrfanView can do it.
http://www.irfanview.com/