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To re-enable the option of displaying resized versions of large images, delete your cookies.
When the image is resized, saving the image by right-clicking it will cause you to save the resized image. However, above the image there is a link to view the image in its original size instead. If you click this, you can then save the original size by right-clicking it.
Alternatively, in the toolbar to the left of the image, you can click "Original Image" to view the original image before resizing.
No, it is not. The resized images aren't strictly resized to fit your screen better. They're also resized so that they take less long to load. The only way to have it possible to right-click on the resized image to save the fullsize one would be to have it actually load the fullsize image, which would defeat the point.
You CAN right-click the Original Image link in the left panel to download the fullsize image.