Use this web utility:

http://www.motobit.com/util/base64-decoder-encoder.asp

Set the output format to binary, then copy-paste the base64 data that follows data:image/png;base64,; your browser will download the file. Rename it to PNG and you're good to go.

The utility base64 reads its input either from a file whose name is supplied as an argument, or from standard input. It never reads its input from a command line argument. In your case, to decode a string stored in a variable, you should supply the string on the standard input of base64.

If you are using Bash, you may use a here-string:

base64 -d <<< "$myImgStr" > image2.jpg

If your shell does not accept here-strings, you can always use:

echo "$myImgStr" | base64 -d > image2.jpg

(Note the doublequotes around "$myImgStr". You should always doublequote variable expansions unless you have a good reason not to.)