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If the image is too big, you can alter its dimensions by entering new values into the Width and Height fields. By default the image ratio is locked, which you can see thanks to a 
button. This means that when you change one of the size values (width or height), the other one will be adjusted automatically.
If you want to freely modify both dimensions, click the 
button in order to unlock the ratio. The button will now change to 
and modification of one dimension will not automatically cause the other one to be adjusted. To lock the image ratio again, click the 
button once more.
You can easily return to original image size by pressing the 
button. This will reset the image size; the original width and height will now appear in appropriate text boxes.
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