Who draws the good diagrams for the motivation of an article
The illustrations that are included inside a journal article are essentially always created by the authors.  This may entail getting somebody else to do the actual graphics work, if none of the authors are skilled in that area, but the design concept is created by the authors.

In some higher-end journals, some articles may be specially highlighted (with graphics about them appearing on the cover, for example).  In these cases, the graphics are typically produced (or at least heavily edited) by the journal's design staff.  The authors are involved in decided how best to illustrate their work, but the journal professionals create the actual images.