For the photos on Google+, you can click them and view their enlarged versions in new windows, such as the following:
For the enlarged photo on that window, you can add tags to people, leave a comment, rotate it, and so on, mostly like what you can do with the photos on Facebook.
This is a cool way for you to preview a photo, but if you don’t want to open a new window, you can check out +Photo Zoom, which is a Chrome extension for you to zoom photos on Google+ in the same page.
After installation, refresh the Google+ tab or even restart your Chrome browser, and hover the cursors over the stream photos, then you will see their enlarged versions, such as the following:
By the way, the Chrome extension +Photo Zoom is only workable for those photos (including profile pictures) on your Home and Profile stream pages, but not on the Photos page.