(I wrote:)
Really? I did not know that, are you sure it can contain vector graphics? I thought it was just bitmap; a replacement for GIF basically. I can't find anything that suggests otherwise.
The reason I suggested it is that I find it surprisingly compact for schematics and the like, even at quite high resolutions. And PNG is lossless of course. As a rule I use JPEG for photos, and PNG for anything computer-generated (where I want an image I can use anywhere that is).
A vector format like PDF or eps or SVG should be used where possible but it may not be in Word.