While updating content with graphics used as CSS-background, editor in HTML mode destroys code.

As long as I'm saving content in WYSIWYG mode, everything is OK. But when I go to HTML mode and then press 'Update', then code is destroyed and background is not displayed.

Sample code changes:
<td height="47" style="background-image: url('http://149.156.81.226/tmp/file.php/36/layout/ramki/pamietaj_01.jpg');"></td>
<td height="47" style="background-image: url('http://149.156.81.226/tmp/file.php/36/layout/ramki/pamietaj"1.jpg');"></td>

Problem occurs both on Chrome and Firefox.

Renaming graphic files is not an option since similar style is used in too many courses. Now all our old courses cannot be edited properly.