There are many applications available around which can be used to resize images in Mac OSX 10.6. However most of them are paid. So you are looking for a free solution to the problem. No worries as in Mac OSX we already have this functionality and it can be done in simple clicks. Here is step by step instructions.
1. Open the image in perview (in Mac OSX it is a default image viewer)
2. Click on Tools
3. Select the image resize from the drop down menue
4. Here we go now you can set the size as you want
5. Now save image as from the file menu