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However, at the printing shops the sizes are preset. So the canvas size needs to be 24"x36". So, once happy with the picture used photoshop to setup the picture as follows,

Change Image Size to Inches Units and adjust to 19.25" x 2.75" and 300 Resolution

(pictures below)

Then adjust canvas size to 24"x36",

(pictures below)

Finally I added border so the print people don't get confused and try to adjust the photo,

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Amateur Attempt

I pulled up Photoshop 7 and did my own samples. Not being an expert, the background is too dark so we can't see my daughter's arms properly,

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