There was a presentation on the reporting engine QWeb by Simon Lejeune during the 2014 Opendays (Video | Notes).  Also more information on QWeb can be found in Odoo's 8.0 Documentation.

I believe you want to use: <img t-att-src="http://localhost:..."/>

If you haven't change the RML Header setting in the Company, you can find example there.  The image tag can only accept images that are stored in database.  Example in the standard RML Header is: <image x="1.3cm" y="27.7cm" height="40.0" >[[ company.logo or removeParentNode('image') ]]</image>, which takes the company's logo stored in the database.  If you need images other than the Company's Logo, you can attach it to ir.attachment and use setHtmlImage.  Here is the explanation of setHtmlImage: setHtmlImage(id, model=None, field=None, context=None),  returns the value of field as specified by given database id in the given model.  If model is not specified, it will return the value of field datas of ir.attachment.  Or, alternatively you can use report_webkit's ir.header_img and make use of it's get_logo_by_name method to obtain the image by name.