A user asked in a Google Web Search Help thread, what does Google do with the images you upload to the search by image feature?
What if you are uploading private photos just to make sure those images are not on the internet. What does Google do with those images?
Kelly from Google answered this question in a bit more detail in the forums, saying:
When a user uploads an image or provides an image URL to search by image, we treat that image as the user’s query and will log a thumbnail sized version of it. The reason for this is that our recognition engines are trained using a large corpus of real world images. We also retain image queries for the same reasons we retain text queries: to fight spam, improve the relevance of search results, etc.
Like text queries, your image queries are not public.
Third-party vendors cannot access this data. As mentioned before, the thumbnails are fed into a larger pool of images that help us improve the quality of this feature.
So you'd have to assume the images are stored for as long as they store their log files, which is "anonymizing IP addresses after 9 months and cookies in our search engine logs after 18 months." But there is nothing specific if those images are ever deleted.
Forum discussion at Google Web Search Help.