If you want to upload the image in the Mendix I would recommend to use the image uploader and save the image in a seperate entity.
If the POST is to an external service you can either send the image as string in base64 format or expose the images via an API and use the URL as string.
For the base64 string you can use the Base64 Java actions from the community commons module (https://docs.mendix.com/appstore/modules/community-commons-function-library).
When you create an API, you have to think what the resource ID should be and how to handle file extensions (e.g. .../api/v1/media/images/1.jpg).
First you download the community commons module (https://docs.mendix.com/appstore/general/app-store-content).
Then in a microflow you use the Java action Base64 Encode File with the image as input, the Java action returns a Base64 string of your image.