Tips & Tricks
Preview is one of the most underappreciated apps on a Mac; especially in later versions of OS X, it became hugely powerful, and even for us at MacFormat, it does much of what we’d traditionally use a more elaborate and expensive application such as Photoshop for. Do yourself a favour: open an image in Preview and poke around the app’s menus and interface to see what it can do.
For example, you can crop your image. Draw a selection with the regular Rectangular Selection tool then either hit Command+K or choose Crop from the Tools menu. Alternatively, show the Edit Toolbar and make a more complex selection either with the Instant Alpha tool (like in iWork) or use the Smart Lasso. With this tool, you draw as carefully as you can around the outline of the object and then Preview works out as closely as it can where the edges are. In either case, cropping will, if the image isn’t already a PNG, convert it so that you can have the thing you’re cutting out on a transparent background. (You might need the Invert Selection command, too!)
You can also resize images, and even do some tweaks to the colours with the Adjust Color pop-up. Select Adjust Color… from the Tools menu, and you get some handy sliders and a histogram to help you tweak things. Plus, hit the backtick symbol (to the left of Z on a UK Mac keyboard, and to the left of 1 on a US Mac keyboard) to bring up a loupe so you can see what’s happening at 100% as you make changes.