Preview's annotation tools for PDFs are fairly well known. You can add an oval, rectangle, note or link to any PDF document simply by going to the Tools menu and choosing Annotate. However, if you have a non-PDF image open, like a JPEG, GIF or PNG, you'll find that the annotation tools are greyed out. This might lead you to think that you can't annotate images, but in fact there is an easy workaround.

With an image open in Preview, go to the View menu and choose Customize Toolbar. In the window that appears, drag the Annotate button up onto your toolbar, then click Done. Now, the Annotate options in the Tools menu may be inactive, but the toolbar button still works.

Just Click and hold on the Annotate button to bring up the different annotation options. You'll notice that the link option has been replaced with line, which makes sense since unlike a PDF, there is no text to link in an image. Once you have selected your tool, just click and drag on the image to add the annotation.

As usual, you can choose Show Fonts and Show Colors from the Tools menu to edit the look of your annotations. In addition, the - (minus) and + (plus) keys will change the thickness of your lines.

There are a few little quirks with this though. Firstly, I sometimes had problems re-opening the Fonts window after closing it. If this happens, quitting and reopening Preview seems to fix it. Secondly, note that saving the file makes the annotations permanent, meaning you can't edit them. Thirdly, the options like shadow and underline in the Fonts panel don’t seem to work. The changes happen, but revert back to normal as soon as you close the Fonts window. Finally, you can't copy and paste your annotations like you would do to other objects. Instead, this just copies and pastes the entire image.

These small flaws aside, this is a great way to quickly make rough changes to images without having to resort to third party software. It's great for adding quick notes to screenshots, and I even made my Twitter Background using it.