Can graphics created with Wolfram Mathematica be freely used and distributed?
There is nothing wrong with making public an image created with Mathematica; that's the whole point of the software. The license restricts your rights regarding what you can do with the software package itself (no copying, reverse-engineering, selling, etc), but not the content you produce using the software. If that was the case, no one could use the software to do anything.

You may, however, be restricted by the journal to which you submitted your article. The publication holds all rights to your article, including the figures. You may retain some rights to using the images in your own work (e.g., presentations); you'd have to check with your specific journal.