Fix image miniature generation

I made two simple fixes:
* If a content has a field named "image" that isn't an ImageField (type != image or blob), now the viewlet do not breaks
* Added exception handling also for TypeError in get_content_image method. I had cases with custom contents, that generates a TypeError and was not correctly handled.
I think you can write this in only one except as:

```python
except (AttributeError, TypeError):
    img = None
```