The convert program is a member of the ImageMagick suite of tools. Use it to convert between image formats as well as resize an image, blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more.

# To convert an image from JPG to PNG:
convert <image.jpg> <image.png>

# To scale an image 50% it's original size:
convert <image.png> -resize 50% <image2.png>

# To scale an image keeping the original aspect ratio to a maximum dimension of 640x480:
convert <image.png> -resize 640x480 <image2.png>

# To horizontally append images:
convert <image1.png> <image2.png> <image3.png> +append <image123.png>