How can I insert an image in API?

If you want to send image as binary data, you can use res.sendFile function.

Below is an example that checks that req.user exists and then sends either JSON with a link to the image or the image itself. If the image is small it may be better to send a base64 encoded version.

app.get('/info', function(req,res){
    if(req.user){
        res.status(200).json({
            'imageName':'some image',
            'imageUrl':'/someImageUrlOnlyForAuthorizedUsers.jpg'
        });
    } else {
        res.status(401).send('Authorization required!');
    }
});

app.get('/someImageUrlOnlyForAuthorizedUsers.jpg', function(req,res){
    if(req.user){
        res.sendFile('./mySecretImage.jpg');
    } else {
        res.status(401).send('Authorization required!');
    }
});

If you want to create an object with the information in base 64 and with this insert it in the frontend, what you have to do is:

const fs = require('fs');

let buff = fs.readFileSync('/someImageUrlOnlyForAuthorizedUsers.jpg',{encoding: 'base64'});
let base64data = buff.toString('base64')

Then you can return this object in the api and once you capture it in the frontend I'll show you an example:

<img src="data:image/png;base64,"+base64data">