I wanted to state the source of the illustration I made by myself. The illustration shows the structure of the company, which is based on my research and a number of other sources. What is the best way to state the source? Can I just write Author or is there a better alternative?
There are three options:
1) It is yours and you made it from scratch especially for this publication. You do not need to cite anything. If reviewers or the editor ask, mention that it is yours.
2) It is yours and you based it on some other illustration X that was published by someone else. Then mention that you based it on X and cite X.
3) It is yours and you published it before. If you are sure you are allowed to re-publish it by the original publisher then cite the original publication. If you are not 100% sure, consult the original publisher about their rules for re-publishing figures.
Cite it like it's not yours, i.e. the same way you would cite anything coming from anyone else.
If it's yours, there is no need for citation!
If it was published before, you should cite the first publication of it (of course while respecting all copyright rules as always when you are using illustrations).