Elements hidden by the hide on device feature aren't gone forever; At any time, you can see all of the hidden elements on a page by pressing the 'show hidden elements' option (usually in the bottom-right corner of any page on which an element is hidden).
Un-hiding an element
Once it's shown, you can un-hide hidden elements by pressing it; a pop-up will display prompting you to un-hide the element.
Unhiding an element isn't revealing my image
If you're using the unhide element feature and it's not revealing the image on your site, your image might need to be re-added and uploaded. See this article for more info.