>>1124
You may use any program which supports bicubic or Lanczos resizing. They won't ruin any information which you have on thumbnail.
However, they won't introduce any new information either and your pic will still have resolution of a thumbnail just without square pixels showing.
If you want to upscale an image and make it look like it is it's own resolution you should redraw the textures and details manually...
... or search for a bigger image with Google.
The tool from >>1130 is suitable for solid-painted images (partcularly from cartoons), it won't be much better than upscaling if applied to a photograph.