Hey Vonnagy,
I think I know where the source of the issue is: the “WP Media Gallery” tab in the “Background Slider/Image Source of Images” settings box refers to the native WordPress media gallery (i.e. the “Media” admin menu), not to a third-party plugin; this is mentioned on page 8 of the theme manual, yet I will make sure that it is explained in more detail in the next versions. When you choose the “WP Media Gallery” tab, the slider will source images from the image attachments of that particular page, i.e. every image you upload either via the “Add Media” link above the content editor, or as the featured image. Note that for the images to be included in the slider, you do not need to actually insert them into the page (i.e. pressing the “Insert into page” button) – simply uploading them from that page is enough; also, for the images to be displayed in the slider, they need to be attached to the page, i.e. uploaded from that page (that means that if you choose an existing image form the Media Library tab while in the image upload pop-up, it will not appear in the slider for that page).
Sincerely,
Andrii / Satori Studio