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That is due to the image software which you are viewing .Please open the same png in browser and see whether you can create the same issue or not.
I have faced the same issue with pdf .The problem is due to the pdf software.
So the same thing is happening it is due to the software.
When manually downloading images from Tableau Server I've found that the random lines you mentioned appear when using Internet Explorer. However, when using Google Chrome the random lines no longer appear, and the downloaded image quality is higher. Not sure why this is, but it is consistent behavior on my and my co-worker's computers.
Try downloading the image in IE. I faced the same issue in chrome in which I got random coloured lines. But the images downloaded via IE had no issues.