Changes:
Added maximum width and height to the variables and settings file
Restored clicking and dragging outside the image to start selection
Added checkbox, menu item and F12 shortcut to toggle image visibility
Added link to the tutorials page
Added the currently open filename to the title
Added dragging and dropping an image onto the RichTextBox to load it
Added pressing escape while dragging/moving selection rectangle to cancel changes
Cleaned up and refactored JMSelectablePictureBox a little
Added holding shift while selecting to constrain it to a perfect square

I’ve also updated the tutorial for the new interface:
http://ascgendotnet.jmsoftware.co.uk/ascgen-dotnet-tutorial-1-creating-an-ascii-image