I have tried the latest development created from source on Ubuntu 11.10 and the problem is exactly the same.
Notes:
- before step 3 I have created a print screen pressing <Print Screen> key, 
- after step 3 there is additional question if I would like to expand canvas, so I selected "Expand canvas" button.

I have run Pinta from terminal. Terminal output is.
*** Start of terminal output ***
Stacktrace:

  at (wrapper managed-to-native) Gtk.Application.gtk_main () <0xffffffff>
  at Gtk.Application.Run () <0x0000b>
  at Pinta.MainClass.Main (string[]) <0x0059b>
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <0xffffffff>

Native stacktrace:

	/usr/bin/mono() [0x80e126c]
	/usr/bin/mono() [0x812046c]
	/usr/bin/mono() [0x805fe7d]
	[0x4002040c]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(+0x2483d3) [0x410723d3]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(+0x23b11a) [0x4106511a]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(+0x23bb90) [0x41065b90]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(+0x252804) [0x4107c804]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(+0x2561c2) [0x410801c2]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(+0x138c52) [0x40f62c52]
	/usr/lib/i386-linux-gnu/libgobject-2.0.so.0(+0xb68d) [0x4179168d]
	/usr/lib/i386-linux-gnu/libgobject-2.0.so.0(g_closure_invoke+0x1ac) [0x41792c3c]
	/usr/lib/i386-linux-gnu/libgobject-2.0.so.0(+0x1f838) [0x417a5838]
	/usr/lib/i386-linux-gnu/libgobject-2.0.so.0(g_signal_emit_valist+0x589) [0x417ae599]
	/usr/lib/i386-linux-gnu/libgobject-2.0.so.0(g_signal_emit+0x33) [0x417ae8f3]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(+0x272553) [0x4109c553]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(gtk_propagate_event+0xce) [0x40f60eae]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(gtk_main_do_event+0x2c0) [0x40f61240]
	/usr/lib/i386-linux-gnu/libgdk-x11-2.0.so.0(+0x57888) [0x412ea888]
	/lib/i386-linux-gnu/libglib-2.0.so.0(g_main_context_dispatch+0x1df) [0x4181d25f]
	/lib/i386-linux-gnu/libglib-2.0.so.0(+0x43990) [0x4181d990]
	/lib/i386-linux-gnu/libglib-2.0.so.0(g_main_loop_run+0x14b) [0x4181df9b]
	/usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0(gtk_main+0xaf) [0x40f5ffcf]
	[0x441bfb38]
	[0x441bfafc]
	[0x4075d354]
	[0x4075d567]
	/usr/bin/mono() [0x8064afc]

Debug info from gdb:

Could not attach to process.  If your uid matches the uid of the target
process, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try
again as the root user.  For more details, see /etc/sysctl.d/10-ptrace.conf
ptrace: Operation not permitted.
No threads.

=================================================================
Got a SIGSEGV while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries 
used by your application.
=================================================================

make: *** [run] Aborted

*** End of terminal output ***

It looks to me like a mono crash. On Windows (see attachment from first post) I also see different window then usual Pinta window crash. On Pinta I have executed "mono --version" command and got output:

***
Mono JIT compiler version 2.10.5 (Debian 2.10.5-1)
Copyright (C) 2002-2011 Novell, Inc, Xamarin, Inc and Contributors. www.mono-project.com
	TLS:           __thread
	SIGSEGV:       altstack
	Notifications: epoll
	Architecture:  x86
	Disabled:      none
	Misc:          softdebug 
	LLVM:          supported, not enabled.
	GC:            Included Boehm (with typed GC and Parallel Mark)
***