Producing images in different sizes

This is off-topic, but someone in the group may have a good idea, so I'll ask it here. Please also reply to my email - ray@non-aol.com -- thanks. I am storing several detailed images at about 400x400 pixels (inventory for an art gallery). I show a page with thumbnails of the images. When a site visitor clicks on a thumbnail, I open a separate window to display the detailed image. Presently I am storing two copies of each image - full size and thumb. My objective is to store only one copy of each image - full size. (The rationale behind this is a complicated story involving old gallery owners, digital cameras, rudimentary image editing skills, etc.) I want the server to read the full size image, but resize it down to the thumbnail size before sending it, thus saving transmission time. Are there any server-side applications that can handle this? Thanks for your suggestions, --- Ray Paseur - www.non-aol.com