Just go to the Yoast Plugin, Titles & Meta tab, then under Post Types find "Symbiostock Images". In the meta description template part put "%%excerpt%%" (without the "") and it will auto generate it for you. Just be aware, it doesnt visually show up in the meta description box on the image edit page, it just applies it in the background. You may need to tick "Excerpts" to be shown on the Screen Options dropdown on the image edits page.

Hope that helps.

I'm a little bit confused about this. Hoping someone can clarify.  If I add %%excerpt%% to the meta description in Symbiostock Images (Yoast Seo templates) I can see that the there is a meta description being added (by looking at the number of characters) but if I then add any additional information to the content, it is no longer dynamically added to the snippet as it was before adding %%excerpt%%. Whatever title and caption the image has in it's metadata when I upload it becomes the snippet and meta description even if I change the title or caption later.