One vital fact to keep in mind regarding software to increase image size is they function more effectively when the original is of “borderline acceptable quality” to begin with. This is fact when using software from firms like Alien Skin or the image resizing filters of my program UltraSeps which is engineered for t-shirt screen printing.
Many seem to think these software packages can take a miniscule 3″, 72 dpi graphic lifted from a website to generate a pristine 15″ 300 dpi image, ready for print production when that’s simply not the case. I run into this everyday and need to explain the dynamics of up-sampling to end-users.
Lets use a sign printer as an example, he’s probably using it up-sample images for large format printing. I would imagine his originals aren’t small website graphics and are probably somewhat acceptable as a full size t-shirt graphic for example (12″x12″ at 300 dpi) when factoring in physical dimension and resolution. Therefore, when up-sizing for large format such as a vehicle wrap, they work since this type of output is at lower resolution, well below 300 dpi as its viewed from a distance.
Although this won’t sound incredibly scientific — up-sampling, re-sizing, whatever works well “at times” and some files are more forgiving than others. Its not an exact science.