Please note that Bulk Scheduler is available on the Professional Plan and above.
You cannot upload images directly from your system. It's necessary to save your images as public URLs and mention them in the spreadsheet and we'll directly extract the image from the web.
You can use websites such as
https://ctrlq.org/images/ and
https://imgur.com/ to host your images on the web and generate publicly accessible links for the respective images. Select the image from your storage and upload the image to the above service, to host it. Once this is done, you can copy the Image URL (Copy Image Address or Open Image in a new tab) and paste it in the 4th column (titled 'Image URL') in the Bulk Scheduler CSV/XLS file.
In case you wish to post multiple images, paste the URLs in separate columns as shown below: