Remove blank target from image link in textblock.

This eliminates the "external link" problem in firefox if the theme looks on the targets.
See screenshot:
![externer_Link_weg_bei_Bilder_in_Block](https://f.cloud.github.com/assets/3796673/231686/745f46a2-870f-11e2-84d4-fca0dc8d9d79.PNG)

\cc @maethu