I want to put photos of 3 celebrity sportsmen on the cover of an eBook that I intend to self-publish on amazon and other stores, and I wonder if images that are returned by Google image search with license (under 'search tools') labelled as "for reuse" or "for reuse with modification" are fair game? Can I just use them and be free of any obligations?
Short answer: Maybe.
Longer answer: Yes, you may use images described by Google as being free for "reuse and modification" in your book cover, if...
Google has correctly identified the license for the image,
the license is actually valid (i.e. the person distributing the image actually holds the copyright to the image and has the authority to license it),
the license really permits commercial use,
you comply with all the terms of the license (which will often require you to credit the author, and may require you to release any modified versions under the same license),
the image is not a derivative of some other (not freely licensed) copyrighted work (e.g. a photograph of a painting or a sculpture), and
there are no other, non-copyright issues (such as, notably, respecting the personality rights of any people appearing in the image) that might prevent you from using the image.
Basically, while Google's free image search is a good starting point for finding freely licensed images, you do need to examine each image you want to use individually, verify that it comes from a seemingly legitimate source, find out which license the image is actually released under, and carefully read the license terms to see what you need to do to comply with them, and what potential pitfalls there might be.
In particular, as others have noted, a major potential issue with using pictures of celebrities on a book cover would be personality rights. Most free content licenses (such as those from Creative Commons) are only granted by the author of the work (i.e., for photographic works, the photographer), and only cover copyright. Unless specifically stated otherwise, such licenses do not usually include any sort of permission from third parties, such as people who might appear in the image.
For example, the Creative Commons FAQ says (emphasis mine):
How are publicity, privacy, and personality rights affected when I apply a CC license?
When you apply the latest version (4.0) of a CC license to your material, you also agree to waive or not assert any publicity, privacy, or personality rights that you hold in the material you are licensing, to the limited extent necessary for others to exercise the licensed rights. For example, if you have licensed a photograph of yourself, you may not assert your right of privacy to have the photo removed from further distribution. (Under the 3.0 and earlier licenses, this is implied but not explicit.) If you do not wish to license these rights in this way, you should not apply a CC license to the material where this is a concern.
If there are any third parties who may have publicity, privacy, or personality rights that apply, those rights are not affected by your application of a CC license, and a reuser must seek permission for relevant uses. If you are aware of any such third party rights in the material you are licensing, we recommend marking the material to give notice to reusers.
To reuse such images in a context where personality rights might be an issue (and a book cover would certainly seem likely to qualify), you'll likely need to separately request permission from the subjects of the images, regardless of how the images themselves are licensed.
This is no particulary designer question, it is more lawyers question, but anyway.. According to my experience, you should never use imagery of celebrities without written permission.
Well maybe photograph gave people right to use the photo, but that celebrity person didn't and that is a big difference.
The rights of the photograph usually reside with the photographer, unless commissioned or sold on, in which case it may reside with a publisher/photo agency.