Awk is a command which is an interpreter for a programming language that is aimed at simple text processing. Though it can, it's not really meant to be used to call other commands like the `head` command.

Here, you could have a shell do the work:

    find . -type f \( -name '*.jpg' -o -name '*.jpeg' -o -name '*.png' \
                      -o -name '*.html' \) -exec sh -c '
      for file do
        IFS= read -r line < "$file" &&
           case $line in
             ("#!"*) printf "%s\n" "$file: $line"
           esac
      done' sh {} +

If you wanted to do it with `awk`, with GNU `awk`:

    find . -type f \( -name '*.jpg' -o -name '*.jpeg' -o -name '*.png' \
                      -o -name '*.html' \) -exec awk '
        /^#!/ {print FILENAME ": " $0}; {nextfile}' {} +

With other `awk`s:

    find . -type f \( -name '*.jpg' -o -name '*.jpeg' -o -name '*.png' \
                      -o -name '*.html' \) -exec awk '
       BEGIN {
         for (i = 1; i < ARGC; i++)
           if ((getline < ARGV[i]) > 0 && /^#!/)
             print ARGV[i] ": " $0
         exit
       }' {} +