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From now on, the image frame will keep the same size as the document.
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Add an image
When adding an image to that frame, you must choose a "fitting". Will the image fit proportional (inside) or proportional outside?
We choose to fit proportional outside. Meaning, we'll loose some image, but the frame is always filled, without loosing the proportions.
Because the frame is tied to the size of the document, the image inside the frame will also resize with the document resizing. And, the proportional fit will remain.
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