Is the PNG file format acceptable for academic papers? 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.
Running a journal with Wiley as a publisher, Wiley's instructions state JPEG, TIFF and EPS are acceptable formats. Since many authors supply figures in PDF and PNG I started to pass these on to the type-setter and found that no issues erupted. It is therefore clear that publishers may lag behind with their recommendations while type-setters are quicker to adapt to "new" formats. I imagine the lag is possibly a lack of interest to update what works, but can also be because they work with many type-setters and printers and wish to maintain a largest common set of files that they know all can accept rather than having to constantly adapt to individual services' capabilities. I am fully aware that this thinking is a bit arcane and that any type-setter/printer that cannot handle almost everything has very little potential for survival in today's competitive world.

Anyway. I would suggest asking the journal if it is possible to use PNG in your case, or do as many authors have done in "my" journal, send them in anyway. It may, for example, be possible to send in a JPG and a PNG copy of the graphics asking them to use the PNG if possible. 

As a side point: PNG is in my opinion the best pixel format because of its strong loss-less compression except for photographs, something it was never intended for anyway. Using PNG graphics will keep down the size of final PDFs of the article which is a good goal to have. So in my case, I encourage authors to provide PNG even though Wiley does not.