title: iBooks: my ePub doesn't show PNG images, a question mark is in its place instead
body: <p>I'm making by myself an ePub book. In this book I put some screenshot as a PNG image inside an XHTML page. So in the content.opf I referenced the XHTML page...</p>
  
  <p>Until few days ago it worked: the XHTML with PNG image was showed.</p>
  
  <p>Now I see only a blue question mark at the place of the PNG image preview. But if I double tap onto the question mark, the preview correctly shows the PNG image.</p>
  
  <p>This happens in IBooks and Stanza, but not in GoodReader and others eBook readers...</p>
  
  <p>An tips?</p>
  
  <p>------------------ EDITED ------------------------</p>
  
  <p>Every PNG is ~200Kb and its dimension is ~2500x2500 with 300 dpi.</p>
  
  <p>I don't know if thew problem is the dimension or the image format.</p>