In our 'Authorship, copyright and submission declaration' we have the following definition for copyrighting:
Permission has been obtained for all reproductions from copyrighted sources and is acknowledged in the appropriate place in the manuscript.
In interpreting this statement, we will use the following interpretations:
Reproductions means a direct copy of a diagram from another source (i.e. basically a scan or reproduction of the content), then you need to get permission to use – particularly from a journal or a book or other publication that holds copyright – even if it is one you developed. You need to provide the reference along with the page number below it: 'Source: Reference and page number'.
Where you alter a diagram from another source, sufficiently to claim you have adapted it then you need to acknowledge the source by putting a statement below it: 'Source: Adapted from Reference and page number'.
Where you have used text-based theory to develop a diagram then just acknowledge the source, perhaps something like: 'Source: Developed from reference'.