Apple - How can I get a high-quality screen capture on a mac?

The PNG is losslessly compressed; there's isn't a possible better-quality screenshot.

The image will always have the same dimensions as the screen or window being captured — a 1440x900 screen will create a 1440x900 screenshot.

Today's operating systems are resolution-dependent: everything you see screen is either being shown at actual size (e.g. a 10-pixel-by-10-pixel icon which has actually drawn as 10 pixels by 10 pixels) or has been rendered at some pixel dimensions. So, the size of objects on a computer display is entirely dependent on the pixel density of the display — a higher-resolution screen (or screen capture!) would make everything smaller.

An interesting example of this is Apple's iPhone 4 — its screen has a higher resolution (and pixel density, since the screen is the same physical size) than previous models. Apple had to craft bigger icons to keep everything the same apparent size, and screenshots from iPhone 4 are much larger than older models.

You can change the format of screenshots, and the folder where they're dropped, using utilities like TinkerTool.

Using the terminal you can change the file type to jpeg, which can give you more compressed file sizes.

defaults write com.apple.screencapture type jpg
killall SystemUIServer

Instead of jpg you can also set it to many other file types including tiff, pdf, png, bmp or pict. Try which one works best in your case.