I believe (not a lawyer, so we'd have to ask one of the Wiki Mavens who is) that this is ok to use as the RN website (http://www.royalnavy.mod.uk/terms-and-conditions) says,
'Using www.royalnavy.mod.uk content
www.royalnavy.mod.uk is published under the Open Government Licence, and you can reproduce information from the site as long as you obey the terms of that licence.
The material featured on this website is subject to Crown copyright protection unless otherwise indicated. Read the Crown copyright page for more information.
In addition, we make much of our information available through feeds to third parties for use on websites or other applications. Please be aware, however, that these are not our products. These applications may use versions of our information and guidance that has been edited or cached. The most up-to-date version of our information will always be that available on the www.royalnavy.mod.uk website.
We don’t provide any guarantees conditions or warranties as to the accuracy of any such third party products and do not accept liability for loss or damage incurred by users of such third party products under any circumstances.'
and if you go to the UK Open Gov't Lisc. website (http://www.nationalarchives.gov.uk/doc/open-government-licence/version/2/) it says:
'Using Information under this licence
Use of copyright and database right material expressly made available under this licence (the ‘Information’) indicates your acceptance of the terms and conditions below.
The Licensor grants you a worldwide, royalty-free, perpetual, non-exclusive licence to use the Information subject to the conditions below.
This licence does not affect your freedom under fair dealing or fair use or any other copyright or database right exceptions and limitations.
You are free to:
copy, publish, distribute and transmit the Information;
adapt the Information;
exploit the Information commercially and non-commercially for example, by combining it with other Information, or by including it in your own product or application.
You must, where you do any of the above:
acknowledge the source of the Information by including any attribution statement specified by the Information Provider(s) and, where possible, provide a link to this licence; ........'
So, I think (can't give a legal opinion) that its ok to use this image, as long as we attribute it the RN.
