Using the Migrate Source CSV plugin to migrate images
You'll need to run a migration of all the files you want first. The file_copy process plugin is going to be your best friend for this part.
To get you started,

id: image_migration
destination:
  plugin: 'entity:file'
process:
  uri:
    plugin: file_copy
    source:
      - file_source_uri
      - file_destination_uri


Once you have your files migrated, it's actually very easy to tell your image field about the file

process:
  field_image:
    plugin: migration
    migration: image_migration
    source: image_id


A couple notes:


image_migration refers to the migration that imports your images. Make sure to change it if you use a different name.
image_id needs to be the key from the image migration. It will be mapped to the new file created during that import.


Very Useful Guide