You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query-continue>
    <allfileusages gafcontinue="IMG_0008a1.jpg" />
  </query-continue>
  <query>
    <pages>
      <page pageid="1421" ns="6" title="File:Blue hills.jpg" />
      <page pageid="2138" ns="6" title="File:IMG 0002.jpg" />
      <page pageid="2152" ns="6" title="File:IMG 0003a1.jpg" />
      <page pageid="2153" ns="6" title="File:IMG 0003a2.jpg" />
      <page pageid="2300" ns="6" title="File:IMG 0004a1.jpg" />
      <page pageid="2301" ns="6" title="File:IMG 0004a2.jpg" />
      <page pageid="2141" ns="6" title="File:IMG 0005.jpg" />
      <page pageid="2143" ns="6" title="File:IMG 0006.jpg" />
      <page pageid="2302" ns="6" title="File:IMG 0007a1.jpg" />
      <page pageid="2303" ns="6" title="File:IMG 0007a2.jpg" />
    </pages>
  </query>
</api>