Coudray describes techniques you can use to overcome resolution, color, image-quality, and file-format issues.
Enlarging, or up-sampling, a low-res file introduces all sorts of artifacts into the art. The image-manipulation program (usually Adobe Photoshop) has to guess what values to insert between the existing pixels in order to make up for the missing information. Up-sampled images are not as sharp, nor do they have as much detail, as the original. If the enlargement is up to 125% (1.25x original size), you may stand a reasonable chance of successfully creating a printable image. I have seen some images up-sampled by 200%, but beyond this, you'll definitely have a blurry, choppy image.
To up-sample, choose an interpolation method that gives the best results. In Photoshop, follow this path: >Edit>Preferences>General. With the General Preferences open, choose Bicubic. This is the slowest, but smoothest, rendering method. Even though the image may be slightly blurry, you can further improve the results by applying several passes of Unsharp Masking from the >Filter>Sharpen>Unsharp Masking area. More on this in a moment.
Down-sampling images creates fewer issues. Here, the main problems are files that are way too large or have too much detail in them. In the latter case, the extreme details become unprintable on reduction. When interpolating down, you can use either native Photoshop or Bicubic format with no damage to the file. The only thing you need to be aware of is that when you downsize, you discard information that cannot be recovered. If you downsize, you cannot resize back to the original dimensions without problems.
Another solution to improper resolution is to convert the resolution-dependent raster file to a fractal file. This is a mathematical translation that is similar to resolution-independent vector files. If you use a product like Thunder Lizard's Genuine Fractals Pro, you convert the file to mathematics that are truly independent of size. I have seen 2000% (20x) enlargements of fractal files with virtually no distortion. Likewise, a fractal file can be downsized to 2% (50x reduction) for easy transmission over the Internet. This works great for large-format printers.
Remember, if you have bad art to begin with, it will only get worse when you enlarge it. However, bad art sometimes can be salvaged if it's reduced enough. When in doubt, always ask your customer to give you a file that is larger than what you need.
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