Blog post: http://idevrecipes.com/2010/12/08/stretchable-images-and-buttons

Images for your iOS apps are too big. You’re creating variations of the same image that only differ in width.

A great way to reduce the size of images and reuse images is to use stretchable images.

Smaller image sizes reduce the app size and users will have to wait less for the app to download from the AppStore.

Smaller images also reduce your app’s memory footprint:

Make resource files as small as possible.
Files reside on the disk but must be loaded into memory before they can be used; compress all image files to make them as small as possible.

A stretchable image has 3 parts: A left cap, a one pixel stretchable area and a right cap.
Keith Peters over at Bit-101 has a great image showing this in action:

These images when scaled or resized will draw both caps on either side and repeat the middle pixel.

The most common ways to use stretchable images are:

An entire image stretched by using a 1 pixel wide source image

For example this simple 1 pixel wide image:

UIImage* image = [[UIImage imageNamed:@"1-pixel-image.png"] stretchableImageWithLeftCapWidth:0.0 topCapHeight:0.0]

If we then use this image in a 300 pixel wide image view:

UIImageView* imageView = [[[UIImageView alloc] initWithImage:image] autorelease];
imageView.frame = CGRectMake(0, 0, 300.0, image.size.height);

we get this image:

An image stretched with equal right and left caps

The source image needs to contain both caps with an extra pixel in the middle. So for example this image is 11 pixels wide, 5 pixels for each cap and a 1 pixel stretchable area in the middle:

UIImage* buttonImage =[[UIImage imageNamed:@"button.png"] stretchableImageWithLeftCapWidth:5.0 topCapHeight:0.0]

results in this image when used in a 300 pixel wide image view:

UIImageView* imageView = [[UIImageView alloc] initWithImage:buttonImage];
imageView.frame = CGRectMake(0, 0, 300.0, buttonImage.size.height);

If we create two images then we can set the background image of a button for the normal and highlighted states and get some very nice looking buttons using very small images.

Blog post: http://idevrecipes.com/2010/12/08/stretchable-images-and-buttons