I found a workaround by downgrading to Opera 41 changing my speed dial thumbnail then letting the auto update upgrade Opera back to the latest version of Opera 42 and the image I selected carries over.
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Just updated to 42.0.2393.137 and Speed Dial thumbnails are still broken. I personally noticed back in November 2016 that one of the updates broke the thumbnails.
Either adding a 'New Bookmark' to the speed dial or clicking the heart icon on a saved speed dial link and you get ZERO options for a speed dial tile image. ( I can confirm this on 4 different machines running Win7x64 and Opera 42) Previously you'd have multiple options ie. screen shot of page, image from page, and what you're forced to use now... the stupid Windows looking 'Metro' tile that I absolutely hate.
PLEASE PLEASE PLEASE bring back the option for selecting a screen shot of the page or one of the images on the webpage to use as your speed dial thumbnail