Two options I can think of:
1. Go back to a restore point created before you installed the Image Resizer and then try reinstalling it again (but be certain that doing this wouldn't upset anything else).
2. Create a shortcut to the Image Resizer and put it in the 'Sendto' menu. This isn't so much a cure as a work-around, but it would get the option back into the right-click menu again.
Location of the 'SendTo menu: C:\Documents and Settings\
your name\SendTo
'Your name' isn't the name of a folder. I mean, look out for a folder named bossyboots or whatever.
*EDIT* I just installed XP Image resizer and find that it doesn't have an EXE to create a shortcut from (not found anything so far anyway). It's probably one of those things that integrates into the OS. So, Option 2 may not be an option afterall. I'll keep looking a while.