Example of how to send data through stdin and get results from stdout as buffers

imagemagick-watermark.js

const { readFile, writeFile } = require('fs')
const { spawn } = require('child_process')

function watermark(buffer, logoPath) {
  return new Promise((resolve, reject) => {
    let child = spawn('composite', ['-dissolve', '25%', '-gravity', 'SouthEast', '-geometry', '+10+10', logoPath, '-', '-'])
    child.stdin.write(buffer)
    child.stdin.end()
  
    let chunks  = []
    child.stdout.on('data', function (data) {
      chunks.push(data)
    })
    child.on('close', function (code) {
      resolve(Buffer.concat(chunks))
    })
    child.on('error', reject)
  })
}

readFile('myfile.jpg', function (err, buffer) {
  watermark(buffer, 'mylogo.png').then(result => {
    writeFile('output.png', result , err => {
      if (err) return console.error(err)
    })
  }).catch(err => {
    console.error('Watermark error:', err)
  })
})