Hi everyone, hi Aldert,
I also experiment the same issue, after updating my dev site to WP 3.6 and NGG 2.0... In the Opera console (and also in the Firefox Error Console), I've got lots of DOM errors like this:
[08/09/2013 01:55:21] CSS - http://my_dev_site/
DOM style property
Invalid value for property: width
And errors are for width, height and opacity.
By the way, I do confirm that the issue is related to TimThumb v2. When I disable the Resize option in the NivoSlider widget settings, Nivo works like a charm...
Hope it helps ;-)
(I also hope that the future update will keep the "Crop and resize to best fit the dimensions" option... It's my favorite one :-) .)