Via command-line:

# yum install imagemagick

Or

# aptitude install imagemagick

The mogrify program to resize an image, blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more:

Reduce the size of an image:

$ mogrify -resize 50% image.jpg

Resize without mantaining original aspect ratio:

$ mogrify -resize 640×480! *.jpg

Resize all your JPEG images in a folder to a maximum dimension of 300x300:

$ mogrify -resize 300x300 *.jpg

Via Nautilus right-click option:

# yum install nautilus-extensions

Or

# aptitude install nautilus-image-converter

After the installation, using the Nautilus File Manager, you'll be able to right click on an image, select "Resize image..." or "Rotate image..."