images 'moving' to left by 1/2 a pixel for no reason

in fireworks

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This is excessively annoying - i've been using Fireworks for a few years and never seen this before - I hope someone can help.

I've been making images for image rollovers in Fireworks like this:

crop one image, eg the site logo. Then i increase the canvas size by double vertically, copy the logo and move it down by half the canvas size on the y axis. So it is exactly the same group of images but has been moved down the canvas vertically - no horizontal movement whatsoever - i make sure by just changing the number in the y axis. I didn't notice anything at first and exported a few images into my website folder.

However when i put it into my CSS as an image rollover (using vertical background position change on a:hover) and preview it in my browser there is a slight movement to the left - or possibly it increases in width by 1/2 a pixel. It has happened on every single image i have cropped from the site i am currently designing in Fireworks. I have never seen this before - can anyone please help?

Thank you to anyone who knows the answer.