Decide which boundary matters most
When the image and paper have different proportions, first decide what you want to preserve. You may want the complete composition, a particular outside paper size or a frame you already own. Naming the priority makes the options easier to compare. There is no need to let an automatic printing setting decide the answer for you.
Keeping the full picture usually means choosing matching proportions or leaving unused space around it. Filling an unmatched rectangle usually means cropping. A border is part of the layout; a crop is a decision about which image information to remove.
Work through a real proportion mismatch
Consider a landscape image that is 6,000 by 4,000 pixels, a 3:2 shape. To fill a 20 by 16 inch rectangle, a 5:4 shape, a crop could retain the full 4,000-pixel height and reduce the width to 5,000 pixels. That removes 1,000 pixels across the width. A centered crop would remove 500 from each side, but a deliberate crop could distribute that loss differently.
To keep the whole image at a width of 20 inches, its proportional height would be about 13.33 inches. On a 20 by 16 inch sheet, that leaves about 2.67 inches of height for borders, or roughly 1.33 inches above and below if centered. These rounded figures describe the layout before any supplier-specific trim allowance.
Inspect the edges at the intended size
Check every side of a proposed crop. Look for a subject touching the boundary, a signature, a line that guides the eye or an important patch of empty space. Move between the full composition and the cropped preview. Avoid deciding from the center of the picture alone.
If you add a border, compare its color with the paper and mat you plan to use. What looks white on a bright screen may not match the physical materials. A small proof can answer a color or edge question more directly than another screen preview.
Write a clear instruction for the printer
Record whether the image should fit inside the sheet or fill the chosen rectangle. Include the image dimensions, paper dimensions and where the crop should sit. Ask the printer to confirm how its proof, bleed and trimming process affect the final visible edge.
Keep an untouched copy of the downloaded artwork. Save the print layout as a separate file so you can return to the full composition for another size. The first crop does not have to become the only version of the image you keep.