Here is a simple guide how to change the default size of the thumbnails that WordPress 2.5 creates when you upload an image.
Unlike all previous version, WordPress 2.5 has a very handy new feature. It lets you specify custom thumbnail size for your uploaded images.
To adjust the thumbnail size to your liking, go to Settings > Miscellaneous and change the thumbnail width and height under Image sizes section. Let’s set the width and height to 100×100 for this example.
Also, make sure the Crop to size option is selected, and confirm the changes.
(With Crop to size checked, you don’t have to worry about what size your original image is. It will be adjusted to fit your specified size)
That’s an easy way, right? Gud lak!