To my ear,
"Description of new image, modified from Stockfisch et al. 2017"
sounds more correct, as you are taking something from the original owner of the image and modifying it. Here "from" is used as a word to "indicate the source or basis."
As a matter of general English usage (which may be different from the accepted expressions in your specific field), "modified after" means "changed subsequent to something which came before," which makes less sense in this context (the modification and use of a previously published image).
That said, using "after" is not incorrect, as it can mean "in the characteristic manner of" or "in imitation of," such as "writing after the manner of Hemingway" (definitions and example from Merriam-Webster).
More importantly, some academic writing guidelines require the use of "after." In the guide which I arbitrarily selected it states, "When the source has been modified, the word 'after' is placed before the author's name in the citation," and their example is, "Table 4.1 Site details of the watershed catchment (after Smith et al., 2002)." (From the "Suggested Style Guide,
North Dakota State University Agricultural and Biosystems Engineering, Revision Date: 18 October 2016," no author given,
In conclusion, if you have the freedom to choose the style of your citations, "modified from" sounds more natural as a matter of general usage, but should the university where you teach or the academic publication have its own guidelines, then of course you should follow them.