Screenshots keep redirecting to Mail app?
Hope this helps someone else. i spent months and months being annoyed by this but mainly because i dont do it very often. My problem was specifically because I had a new macbook pro with touchbar.
It may be that .png files on your Mac have Mail set as the default app. not mine but click on images and check "Open With"
Related to the above it could be a jammed pref pain for controlling default apps.
But also, on screen print, on a MBP watch the touchbar.
Even more annoying for me was it replicated something on the prefs to an iMac and thats where i was really doing screen prints and trying to resolve this issue.
hope this helps someone else.