By default, when you insert an image, it adds a textbreak after its content.

If we want a simple border around an image, we wrap the image inside a table->row->cell

On the image with the red border, even if we set the row height to the height of the image, the textbreak is still there:

 

But if we set the rowStyle 'exactHeight' to true, the real row height is used, removing the textbreak:

 

In this example, image is 250px height. Rows are calculated in twips, and 1px = 15twips.

So: $table2->addRow(3750, array('exactHeight'=>true));