[email protected] Gives 502 only for SOME images

There are some low limits in Lambda, especially in [email protected] on the response size. The limit is 1 MB for the entire response, headers and body included. If lambda function returns a bigger response it will be truncated which can cause HTTP 500 statuses. See documentation.

You can overcome that by saving result image on S3 (or maybe checking first if it's already there), and then instead of returning it just making a 301 redirect to CloudFront distribution integrated with that bucket - so image request will be redirected to result image.

For example in node.js with Origin-Response trigger:

'use strict';
exports.handler = (event, context, callback) => {
    // get response
    const response = event.Records[0].cf.response;
    const headers = response.headers;

    // create image and save on S3, generate target_url
    // ...

    // modify response and headers
    response.status = 301;
    response.statusDescription = 'Moved Permanently';
    headers['Location'] = [{key: 'Location', value: target_url}]; 
    headers['x-reason'] = [{key: 'X-Reason', value: 'Generated.'}]; 

    // return modified response
    callback(null, response);
};

Version for simple Lambda Gateway (without Origin-Response, replaces headers):

exports.handler = (event, context, callback) => {
    // create image and save on S3, generate target_url
    // ...
    var response = {
      status: 301,
      headers: {
        Location: [{
          key: 'Location',
          value: [],
        }],
        'X-Reason': [{
          key: 'X-Reason',
          value: '',
        }],
      },
    };
    callback(null, response);
}