Direct file system access is allowed only in JS(Node.js) scripts running in Backendless Hosting. Custom business logic code running in API Engine can work with files only via API.
Yes, you should be able to do it that way. To do this, deploy your node.js script to the web/scripts folder. It must follow the expressJS pattern of accepting a request and returning a response. Any dependent node modules should be deployed to the web/scripts/node_modules directory.
In your script, you would obtain the data sent by the client from the “request” object and then do your image conversion magic.
ImageIO does a lot of things which are not permitted by the sandbox environment where custom code runs. For example, this is only one of the security exceptions raised by things that class does: