Image reference is an option to load an external image, pick colors in the image and apply the picked colors in the colorway. By default the section is open and ready to load an image. Else, the section can be closed or open by dragging the "...." icon below the image reference section.
Clicking on the upload button you can search and select the image (unsupported for multichannel and grayscale). The name is shown next the upload option. The zoom option are available on the left and are same as used in nT Colorations.
Moving the mouse over the reference will enable the color selector. To pick a color, frist select the colorway that you like to modify and then press key alt+double click on the color on the image reference that you have loaded. The color will be applied in the first channel of the selected colorway. Keeping picking colors will apply to next channels continuously.