It is a little difficult to debug in the browser. The Amasty extension is using ajax to hit your server for product image sets when each size-button is clicked, so the images are not available in the browser for inspection until they are requested.
And the returned HTML is
<img id="amasty_zoom" data-zoom-image="https://www.cammaleao.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/images/catalog/product/placeholder/image.jpg" src="https://www.cammaleao.com/media/catalog/product/cache/1/image/265x265/9df78eab33525d08d6e5fb8d27136e95/images/catalog/product/placeholder/image.jpg" alt="Ankle Boot Oxford Wirth Couro Taupe-39" title="Ankle Boot Oxford Wirth Couro Taupe-39" />
And the return HTML is
<img id="amasty_zoom" width="265" height="265"
data-zoom-image="https://www.cammaleao.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/a/n/ankle-boot-oxford-couro-taupe-wirth-1_1.jpg" src="https://www.cammaleao.com/media/catalog/product/cache/1/image/265x265/9df78eab33525d08d6e5fb8d27136e95/a/n/ankle-boot-oxford-couro-taupe-wirth-1_1.jpg" alt="Ankle Boot Oxford Wirth Couro Taupe" title="Ankle Boot Oxford Wirth Couro Taupe" />
There shouldn't be too many surprises there.
I think you need to study product id 3333 very closely, looking at the gallery images and comparing it to product id 2687.
It looks like some settings are wrong in the way the product images are set up for that product id 3333.
Also, you can actually just put those URLs directly into your browser:
It will show the images that are returned (or the placeholder in the case of 3333). You can add different Product ID numbers into the URL to test other products.
You mention clearing caches but please also ensure your indexes are updated.
If you are certain that the product are setup correctly then I suggest you switch to the default magento theme and disable the Amasty extension (admin->system->configuraiton->developer->advanced [disable the module]) to confirm that Magento can access the images. Then turn the Amasty extension back on and see what happens.