Hi Benjamin!
Chiming in to build on the replies from Jess and Abhisek. In my testing, using a pixel measurement similar to what Jess mentioned lead to the background image filling the canvas in the designer, though I don't see a way to resize it directly when you add it from the background section at the top of the designer. Typically the pixel recommendation for a letter sized piece of paper will actually depend on the DPI (dots per inch) of the image being used (larger for higher DPIs) - so to Abhisek's point, that is a closer recommendation when printing high quality images, however I found it displayed too large as a background image directly in the designer. When you actually print the certificate (like save the PDF), the background will resize to fit the page, so you could kind of combine the two solutions provided, using a smaller pixel count to line things up within the designer then replacing with a larger pixel count for a higher quality image. I personally didn't notice too much of a quality difference when testing different sizes but it can depend on the image being used, so you can definitely test around with it to see what looks best for your image!
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Terryn MacDonell
Solutions Specialist
Cvent
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