Hi, 

using Shotwell 0.18 and the 0.20 (via ppa:yorba) on 
Linux Mint 17. This is on a Lenovo, Intel Core i5, 4 GB RAM Intel
Graphics with shared memory. The System is installed without swap partition.

The following crash can be reproduced
easily by the following steps: 
1. open shotwell
2. double-click on a picture to open the viewer-mode
3. browse through the pictures 
4. shotwell crashes after approximately 55 pictures

Since the consumption of memory increases with each picture, the
RAM fills up quickly - this seems to lead the the crash. 
I used the Gnome-System Monitor to check the RAM consumption. 
It seems as if shotwell does not clear pictures from RAM.

The following error message is provided after the crash when
shotwell is started via the terminal: 
L 2346 2014-09-21 19:44:38 [CRT] orientation_rotate_pixbuf: assertion 'GDK_IS_PIXBUF (pixbuf)' failed

(shotwell:2346): GdkPixbuf-CRITICAL **: gdk_pixbuf_scale_simple: assertion 'GDK_IS_PIXBUF (src)' failed
L 2346 2014-09-21 19:44:38 [CRT] dimensions_for_pixbuf: assertion 'GDK_IS_PIXBUF (pixbuf)' failed
**
ERROR:/build/buildd/shotwell-0.20.0/src/Photo.vala:3436:photo_get_pixbuf_with_options: assertion failed: (scaled_to_viewport.approx_equals(Dimensions.for_pixbuf(pixbuf), SCALING_FUDGE))
Aborted

My interpretation is that the crash occurs because of full memory - and the crash report is just reporting a problem that is caused by this. 
I observed this problem with version 0.18 and upgraded to 0.20 via the 
ppa:yorba repository. The problem is still the same. 

Feel free to contact me if you need further information. 
Keep up the good work.