this is interesting, i'm not having to logout and login for the changes to take place
i just copy in the new png file, authenticate to replace the existing one, then i F12 to bring up the dashboard, hold down option to close the Calculator widget, hit the + sign in the bottom left, and drag the calculator back out onto the dashboard, and it uses the new image
for mnstr_trd_sd:
open your main hard disk, usually called Macintosh HD i think, then open the folder Library. scroll to the bottom, there'll be a folder called Widgets. open that and find the Calculator one. hold down the control button and click on the icon for the Calculator file, it'll pop-up a menu, choose Show Package Contents. that opens a new window
find the folder Images
then, download the pic from this forum that you want to use (ctlr-click and save to disk, or just drag the image to the desktop) rename it as Calculator.png
then drag it into the Images folder - it'll tell you it can't be put there, unless you authenticate, click authenticate, click Replace, type in your admin password, then bring up the dashboard, close the calculator if it's open, hit the + in the bottom left corner to bring up all the widgets, drag the calculator out and it should be use the new skin after a second or two
hope this helps