When you add an offer to your review site a screenshot will be automatically generated. If you want your own image or picture instead, go to "edit offer" and select the offer you want to edit. Next to the screenshot you see a list of buttons. When you click on Screenshot, a dropdown menu will open. Here you can upload your own image. If you want to generate a new screenshot (because for example, the website of the offer has changed), you can also select this in the dropdown.
A new screenshot is rendered by a job that runs every ten minutes, so it is normally visible within a few minutes - both in the CMS and on your review site. You do not need to clear your browser cache: the image gets a fresh address every time it is regenerated.