When I have to use Microsoft Word for writing a paper, I tend to export most my graphs and charts into the PNG image format since I find it the most convenient due to its smaller size (than TIFF) and higher quality (than JPEG).
However, when I wanted to submit papers for conferences, none of them (three in total) listed PNG as an acceptable format for figures. Two of them requested all figures in either JPEG or TIFF formats, and the other asked for (direct quote):
Use the following standard image formats: BMP, JPG, JPEG, TIF, WMF, or EPS.
Is there something inherently wrong with PNG figures for representing scientific data that I'm not aware of? Should I quit using it for everything?
I do understand the advantage of vector graphics. I use PDF to store the originals, which I then convert to PNG for use in Microsoft Word. As suggested in Peter's answer, I've sent an email to the support address for that journal to ask for more information.