Here is a patch which addresses your problem. I have tested it with the following line in the report structure file:
$structure['multi1']['fields'][1][] = array('field' => 'picture',
'width' => 75,
'notext' => 'y');
Note that <imagewidth> and <imageheight> are supplied in the field's entry in the $fieldspec array.
The 'notext' entry is optional. If it is not there the field name for the image will appear after the image.
Note that each line which is output from the 'multi*' area will be as deep as the deepest cell.