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I have downloaded an image that someone has uploaded to my website with a public upload form.
My intention was to copy the file in a virtual machine and see the image from there; unfortunately, on ubuntu there is something called "thumbnail"...
So, thumbnail was created and I can see the image in the file explorer, but I've never clicked on the file.
I run on standard Ubuntu 14.04 (previously lubuntu-desk was installed in this way sudo apt-get install lubuntu-desktop).
It depends on what you mean by safe. Ubuntu has to parse and decode the image file to display it in a thumbnail. So any potential exploit in the decoding code is exploitable when you preview the file.
So from a strict perspective, no you're not safe. In practice, I don't believe there's any known exploits on ubuntu for image files. Historically images tend to be low risk.
If you have reason to be suspicious of a file, turn off the preview feature as described:
