If you root the phone or maybe with ADB Drivers, access the history and extract the images should be possible.
Take a look on XDA-Developers or Google.
From Opera itself I don't know if there is an option.
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There was a chromium switch back in the days like --disk-cache-size="x bytes" it's what he means I suppose. Don't know if it still exists though or if chromium based browsers do have any limitation on the size.