What's the command for ssh to find files which *.png and download them

You can list files of your interest with ssh, then download them with scp:

ssh user@host find /remote_path -iname '*.png' | 
  while read pngfile; do 
    scp user@host:"$pngfile" /local_path
  done

All *.png files under /remote_path will be downloaded to /local_path, but the remote directory structure will not be created (all files will be put in the same directory), so you may have problems if you have two files with the same name in different directories.

Edit

The remote find command line should be corrected in this way:

find /remote_path -iname "'*.png'"

where the internal pair of single quotes are needed to avoid local shell pathname expansion of *, wheras the external pair of double quotes are needed to avoid remote shell pathname expansion of *.

The two types of quotes could be exchanges, and other syntaxes could be used, like \''*.png'\', \""*.png"\" or "\"*.png\"".

If you don't have .png in remote home directory, there is no difference between the two commands, but it is better to be safe.

The fastest way I know is to create tar.gz remotely, transfer it via SSH and untar locally:

ssh user@remotehost 'find /remote/path/ -name "*.png" -print0 2>/dev/null | xargs -0 tar cfz - ' | tar xvfz -

This will work with filenames with spaces and is faster than scp -r and much faster than scp for each file.