Computer Configuration > Administrative Templates > Google Chrome - Default Settings (users can override) > Default search provider
Both
HKLM\Software\Policies\Google\Chrome\Recommended
If DefaultSearchProviderEnabled is on, then setting DefaultSearchProviderImageURLPostParams specifies the parameters during image search with POST. It consists of comma-separated, name-value pairs. If a value is a template parameter, such as {imageThumbnail}, real image thumbnail data replaces it. Leaving DefaultSearchProviderImageURLPostParams unset means image search request is sent using the GET method. The URL must specify the image parameter using a valid combination of the following placeholders depending on what the search provider supports: '{google:imageURL}', '{google:imageOriginalHeight}', '{google:imageOriginalWidth}', '{google:processedImageDimensions}', '{google:imageSearchSource}', '{google:imageThumbnail}', '{google:imageThumbnailBase64}'. Example value: content={google:imageThumbnail},url={google:imageURL},sbisrc={google:imageSearchSource}