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The width and height of the frame are referenced off the top-left corder of the frame itself, unless you change the reference point.
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Make the frame dynamic
The X-position (left side of the frame): -6 px. Why -6? The document has a "bleed" of 6px. This would be usefull if you will also use this document for print output. If you don't intend to output to print, you can set the bleed to 0, and the x-position will be 0px.0 px (see frame Location)
The Y-position (distance from the top of the document reference): -6px, similar reason as X-position0px.
Width: We can now use formulas (by clicking on the formula button) and write page.width+12px (we need to add 12 px, to compensate for the 2 x 6p bleed)
Height: Page.heigt+12 px, same as the document + the compensation for the bleed
From now on, the image frame will keep the same size as the document.
We'll add an image
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