Find all files with the extension .png in the first two levels of the root directory, get the base name of each file, sort them, and print the line number, the base name, and the extension of each file.
$ find / -maxdepth 2 -type f -name '*.png' -exec basename {} \; | sort | awk 'BEGIN { FS="."; OFS="." } {print NR, $1, $2}'