I am a Mac user.

When I make thumbs I do them 2 basic ways.

If I want to crop the picture to show a small portion of the image then I use a program called "GraphicConverter" because it has such a simple method for cutting a piece from a image. Then I make it the size I want using a program called "AutoResizer" to make the thumb whatever size is nescessary.

If I want to resize the complete image to a thumb I can use just "AutoResizer"

"AutoResizer" has the bonus of being able to resize a image to any size and quality and is not a thumbs only tool.

Both softwares (I think) are either shareware or freeware and can be had for little or no cost.