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I need an image that is 8640 pixels wide:
http://i.stack.imgur.com/uoS28.png
Whenever I add something to the 8640 pixel wide image (e.g. in Preview or pixelmator) and save it, it gets cleared and when I re-open the image, I get an empty image.
My question is how I can edit images like this on the Mac (without a Photoshop license)
Example:
First I paste a black graphic into the image and save it

When I then re-open it, the black line is gone again

Did you try pasting something much larger than a line? Or, making sure that Preview is viewing the image at its actual size? I suspect that the line is getting hidden, as it is too small to be seen when the entire image is fit into the window like that. – Kent – 2013-05-28T23:51:14.717
@Kent I did see the black line when I pasted the image into this. Believe me I have tried in various ways. The 1 pixel high image is just as an example for this question. – Besi – 2013-05-29T13:04:53.983