Best practice for scaling images for large banners (print)?

When working on large banners, it’s normal practice to design at scale (commonly 10%) and let the printer output at the final size.

So in your example, you’d place the image at 20 × 10 cm at 300 ppi into a document set up at 10% scale, and the printer would then print it at 200 × 100 cm. That results in an effective resolution of 30 ppi at final size, which is perfectly acceptable for banners and large-format work where viewing distance is significant.

There’s no need to upscale the image in Photoshop. You don’t gain real detail, and large-format printer RIPs generally handle scaling and interpolation better than manual resizing.

Use raster images where necessary, but keep logos, text, and flat artwork as vector wherever possible. As a general rule, aim for about 30 ppi at final output size unless your print vendor specifies otherwise.

Set up your document at the chosen scale (e.g. 10%), place images at their native size, and allow the printer to scale to final output. Export a PDF with no additional image compression, and always check the printer’s preferred specs but this approach is standard practice for banners and billboards (at least in my experience).