User-uploaded images without an account

I’ve used Gravity Forms for this very purpose, it took about 3 minutes to set it up.

The only difference in my case being that visitors were submitting a document rather than an image, but you just specify the allowable file types when creating the upload form field (e.g. jpg, png).

All the major form plugins provide file upload functionality (Ninja Forms, Contact Form 7, etc.).

You might have some coding to do depending on what you then want to do with the uploaded images.