Mobi doesn't use PNG afterall. Slightly better treatment of 'float's.

src/calibre/ebooks/mobi/mobiml.py

@@ -232,6 +232,7 @@ class MobiMLizer(object):
         left = 0
         display = style['display']
         isblock = not display.startswith('inline')
+        isblock = isblock and style['float'] == 'none'
         isblock = isblock and tag != 'br'
         if isblock:
             bstate.para = None

src/calibre/ebooks/mobi/writer.py

@@ -311,8 +311,8 @@ class MobiWriter(object):
         image = Image.open(StringIO(data))
         format = image.format
         changed = False
-        if image.format not in ('JPEG', 'GIF', 'PNG'):
-            format = 'PNG'
+        if image.format not in ('JPEG', 'GIF'):
+            format = 'GIF'
             changed = True
         if dimen is not None:
             image.thumbnail(dimen, Image.ANTIALIAS)

src/calibre/ebooks/oeb/transforms/flatcss.py

@@ -181,7 +181,6 @@ class CSSFlattener(object):
                 left -= style['text-indent']
             if self.unfloat and 'float' in cssdict \
                and tag not in ('img', 'object'):
-                del cssdict['float']
                 if cssdict.get('display', 'none') != 'none':
                     del cssdict['display']
             if 'vertical-align' in cssdict: