Couple ways of creating thumbnails. Not sure whether u want them cropped or shrunk...so...
Depending upon the program you can normally select the image, head to one corner of the image, hit shift and drag the image inwards and it should shrink proportionately. Alternatively you can crop it by hand using a crop tool (duh =P). Or, if you feel really special you can use a program such as photoshop and delineate what you want the pixel length and width to be... and it does the magic for you.

If you need a hand with it, or I explained it horrendously as well is the case... Just ask me to explain it better...