Unfortunately discord slash commands don't have a built-in way to accept multiple image attachments only using one parameter. Your best options (outside of manually defining multiple parameters, one per image) are pretty much:

1. Use a message based command
2. Use a 2 two-step process where you begin the upload using a slash command then send all the images the user wants to upload in a second follow-up message
3. Use a button after the slash command that allows users to upload files