Select all of the thumbnails in the sidebar (with Command-A, or click the first and then click the last while holding Shift). Next, from the Tools menu, choose Adjust Size. You’ll be presented with a dialog containing options to resize to all the common 4:3 and 16:9 screen sizes, as well as the option to define custom dimensions. Images are scaled proportionately and resampled by default.

Save the resized images, and you’re done!
Preview will prompt you to save any changes if you attempt to close the window.

You can also use sips on the command line:
sips -Z 1024 *.jpg
will resample to a max of 1024 px for the long edge.
Note that sips rewrites images in place, so you should do this on a copy if you care.

Batch resize images in Preview is a post from: My MacOS X

1. Open up a PDF or IMAGE on your Mac with Preview.
2. Click on the Annotate button in Preview, located next to the Select button on the top.
3. Now, you should notice a toolbar appear on the bottom on Preview.
4. Use this Annotation toolbar to highlight text, add circles or squares around important text, add hyperlinks, add text, or add colorful arrows.
5. Once you’re done making any changes to the file, make sure to save it.

Here shortcuts:

Annotate images and PDF in Preview is a post from: My MacOS X

Instead of just seeing the regular folder icon in Quick Look, the folder icon will be semi-transparent, with previews of the folder’s contents visible in the Quick Look window, as seen in the image at right. What’s more, the preview images will rotate automatically, moving through each item in the folder (click the image to see a movie of the animation).

To turn this on, quit the Finder (using Activity Monitor, Terminal, etc.), then open Terminal and enter this command:

$ defaults write com.apple.finder QLEnableXRayFolders 1

Relaunch the Finder, and enjoy the new folder X-Ray mode in Quick Look. If you ever want to disable it again, quit the Finder, then repeat the above Terminal command, but change the 1 to a 0.

Source: Macosxhints

Snow/Leopard – Folder content previews in Quick Look is a post from: My MacOS X

if you click Control – or Right-click on a PDF preview and change the display option (e.g. from Single Page Continuous to Two Pages), this setting does not stick between launches. The next time you open a PDF, Preview reverts back to the original display setting.

However, if you change the display setting from the menu (View » PDF Display) the setting does stick, and becomes the new default for viewing PDFs in Preview.

Remember display options for PDF Preview is a post from: My MacOS X