You could check out this thread which has a little app made by one of the members, alternatively you can download one of many freeware image resizers out there.
The MS XP Powertools collection also has a little resizer which can be called up simply by right clicking an image file (once installed).
Just be careful on how many you add into your sig block, more than a couple can get quite large and annoying to many people. Small sig blocks are usually better.