Indexing of WordPress image attachment pages in Google can be considered as duplicate content and might even get your blog punished by Google. If you can see your image attachment pages in Google then your blog is in the danger zone already.
To be on a safer side and to keep your blog secure from duplicate content penalty, you have to redirect the attachment pages back to the original posts. Attachment Page Redirect Plugin can be used for the same. This plugin does not come with any settings, you only need to install it and it will automatically 301 redirect all the attachment pages to the original post. With 301 redirects, you will not only get users redirected but your actual page will also get the link juice from the pages which were not meant to be indexed in Google.
This is just another issue that you should resolve as soon as you figure it out. You can also read more about Google Indexing and you can check this post out too – WordPress Quick Indexing Tips.