Handling image sizes without turning every upload into manual work

Clients keep uploading enormous phone photos to a small site. I can resize the display in CSS, but that obviously doesn't stop the browser from downloading a multi-megabyte original. I want a workflow that makes sensible responsive sizes and a modern format automatically, with the original available if needed. The sites aren't large enough to justify a complicated image service. Are you handling this at upload time, in the build step, or through an image proxy?