How to base64 encode image in linux bash / shell

I'm trying to base64 encode an image in a shell script and put it into variable:

test="$(printf DSC_0251.JPG | base64)"
echo $test
RFNDXzAyNTEuSlBH

I've also tried something like this:

test=\`echo -ne DSC_0251.JPG | base64\`

but still with no success.

I want to do something like this:

curl -v -X POST -d '{"image":$IMAGE_BASE64,"location":$LOCATION,"time_created":$TIMECREATED}' -H 'Content-type: text/plain; charset=UTF8' http://192.168.1.1/upload

I found this http://www.zzzxo.com/q/answers-bash-base64-encode-script-not-encoding-right-12290484.html

but still have had no success.