Image upload in WordPress plugin

Use the file input field on your form for uploading files. When the form is submitted, the file entered in the field is uploaded to PHP’s temp folder. Use the appropriate index in $_FILES to call media_handle_upload(), which would normally move files from the temp folder to somewhere in wp-content/uploads/. You can override the move portion of the function with the ‘pre_move_uploaded_file’ filter. Move the file yourself to where you want it, using the data passed to your callback. Return true to tell WP that moving has been handled and for it to skip that part.
https://core.trac.www.ads-software.com/browser/tags/5.5/src/wp-admin/includes/file.php#L880

Also hook ‘wp_handle_upload’ (applied at line 948 in above linked source code) to update the returned data to reflect the location you moved the file to.