Here's one of my favorite Preview tips. Copy a graphic from just about anywhere, then select 'New from Clipboard' under Preview's file menu. You can then use Preview's tools to crop, annotate, adjust colors, and much more. For example, it's super handy to copy a screen shot from your desktop and make a quick annotation like this:
The real power of this tip, however, comes into play when copying graphics from other programs. Say you have a graphic made up of several elements within a Pages document, and you would like to share the graphic as a JPEG file. First, in Pages, select all the elements and copy them to the clipboard. Here I have selected the four elements (two images and two text boxes) that make up the graphic:
Next, in Preview, select File > New from Clipboard. If you want, do some editing. Then select File > Export... to save in the JPEG format. For my example, here's the result:
That's it - simple and fast. I hope you find this tip as useful as I have.
Note: I've been sharing tips for years on Twitter, long before this site launched. While writing this post, I had a vague memory of struggling to share this tip within Twitter's 140 character limit. Just for fun, contrast the above with this tweet I found from 2009.