What is the order for image selection from a post

I have a website that utilises Get the Image (GTI) plugin to pull a post image into a UDesign theme widget on the homepage. However, I can never work out which thumbnail image will be placed in the widget areas of the homepage, per post excerpt!

See WinAPrizeHome.com.au under slider for 60x60px thumbnails in excerpts.

Your plugin support says:
How does it grab images?
1. Looks for an image by custom field (one of your choosing).
2. If no image is added by custom field, check for an image using the_post_thumbnail() (WordPress featured image).
3. If no image is found, it grabs an image attached to your post.
4. If no image is attached, it can extract an image from your post content (off by default).
5. If no image is found at this point, it will fall back to a default image (not set by default).

With UDesign’s GTI-to-widget integration, I think my options start at no. 4 because I insert images into posts without attaching/linking etc. (no. 3).

How does GTI select an image from the post? Last image inserted? Or what?

If I rely on option 4 above, then how do I set that ON (as you say, it’s off by default)?

https://wordpress.org/plugins/get-the-image/