The submit button shouldn't be there. Otherwise you can make it separate from captcha box. For example under the box of captcha.
You can add some highlights of the task with different color / contrast. For example, the middle of the picture seems confusing for me (transparent in the middle).
I didn't see any pieces of jigsaw, you can make some pieces of jigsaw, drag and drop into the box. That would be help.
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I would recommend you some tips: