The interesting information would be in front of the file name; e.g., I have for screenshot files the following information:
-rw-rw---- root sdcard_r
which shows the file owner and permissions.
What you can try is (as root)
chown root:sdcard_r filename.png
chmod 644 filename.png
rm -f filename.png
This way you will change the owner, then the permissions of the file, and then try to delete it again.