Change Image Number

This plugin shows images for my rss feeds which is excellent however the images it shows are the last images from my posts-i want to show the first image. I tried changing values for “Image Number: To use an image of the post instead of the post thumbnail, enter the number of that image. The word ‘last’ will bring the last image of the post” -but it doesn’t work. WHen i input ‘last’ or any number (example: 1) the rss image disappears.

How can I change the order of the image. Also whats the syntax of entering into the image number field?

Thanks

https://ww.wp.xz.cn/plugins/rss-image-feed/