How to pass an image in a JSON for a HTTP POST?

Hi,

I'm trying to run a HTTP POST request with the following json data

{
"userId":"something",
"userImage":""
}

the userImage is supposed to contain an actual image here. How can I achieve this through Ready API? I did add an image to the attachments tab but don't know how to link it in the request now. Tried cid:image name as well.