To bulk schedule multi-image posts, you should first put your Excel or Google Sheets spreadsheet together as you normally would for the Bulk Uploader tool. However, pay close attention to column G (the image one), as for your multi-image posts you'll have to include the image's URLs separated by a comma. It will look like this:
Your URLs must be public and end on these extensions: .jpg or.png for the Bulk Uploader to correctly process them. Here's an example: https://postcron.com/en/blog/wp-content/uploads/2018/12/artstudio.jpg
Keep in mind that once you upload the file, each row of the spreadsheet will become a different post, so if you have 5 rows, you'll have 5 posts.