Yep it is very difficult to tell and illusion plays a big part of it.

For your reference Shiela I have just had a look at your image above. I can see why someone said it is tilted and also why you say it is straight.
It is because if you straighten the image using the side of the building on the far right then the chimney on the far left is tilted. It has some lens distortion causing this which has bent the outsides of the images outwards. ALL lenses and cameras suffer lens distortion and it doesn't show in most images you just got one where it does. The image is straight - I have not straightened it here at all.

To show you what I mean I have corrected the image as best I can without knowing exactly how much zoom you used on your lens to shoot. Corrections done for Sony F717 in PT lens assuming a focal length of 9.7mm.

Message edited on: 01/06/2006 14:08

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html