open the image, go to Image>size then change it from 256 to 1024. Then, at the bottom of the window, change resize mode from Bicubic to "nearest neighbor," then hit OK.
This method will keep the image from becomming "fuzzy" when you make it bigger.
Panel lines, howevver, will not be 4 pixels wide instead of 1, so you will need to re do all the panel lines to make them 1 pixel (or 2 pixels if they consist of a 1 pixel panel line and a 1 pixel highlight)