Copyright-related advice for graphic designer creating content for academic publications
If the authors of a paper do not specify otherwise, when they transfer copyright of a paper to a publisher, that includes its images. That's what most people do. 

But, during the copyright transfer process, an author can specify


if there are original images of independent value that they'd like to retain copyright to, or
if there are images whose copyright is held by a third party, which they have a license to reuse.


See for example this ACM copyright transfer form.

If you are creating images for papers on which you are not an author, you can retain copyright of your images and just give a license to use in the paper. The authors will have to specify on submission that they are using third party images, and show that they have permission from you to use them.

If you are creating images for your own paper, you may be able to claim them as an "artistic" image and retain the copyright, depending on the publisher.