Photoshop and other image editing applications possess sophisticated engines and algorithms which allow one to increase the resolution of an image via interpolation.
Are there any hard and fast rules about when you should and shouldn't do this? Does your ability to upsample sucessfully (as in the image won't look bad) become increasingly more probably after a certain initial pixel threshold is reached? Is the only way to 'judge' the success or failure of an upsampling attempt by eyesight or are their other metrics one can use to judge the artificially enhanced resolution? Are TIFF images better suited for upsampling than JPEG? Apologies for the barrage of questions but I am new to graphic design in general and am really curious about this particular feature.
One more question - is there a known application which possesses the strongest algorithm / engine / or whatever when upsampling images?