By setting images in your spreadsheet, you can have ContentSync dynamically swap images for each content version.
Image syncing only works when pulling images.
Spreadsheet Format
To sync an image, the image must be set in the spreadsheet for the content key of the image layer.
Raw Image URL
The easiest way is to set the value of the key to the URL of the image you would like to use. Make sure these urls are publicly available so Sketch can access it.
Image Formula
You can also use image formulas on Google Spreadsheets so you can preview the image in your spreadsheet itself. Simply set the value of the cell to =image("https://path/to/image.png").
Layer Support
Symbol Instance Image Overrides
Symbols with image layers or fills may provide image overrides. The key of override for the image will be used to sync the image.