It’s extremely easy to make an image transparent with the help of Preview, the built-in image editing app that comes along with all Macs with OS X. Do note this works best on images with uniform colors in the area you want to become transparent. The more complicated the image and color variation, the more work you will need to do with the alpha tool.

Making a Transparent Image

You can turn any image transparent with Preview, though you’ll find the resulting image must be saved as an image format that supports transparency. If you select a

• Open the image in Preview
• Click the little pen icon in the toolbar to reveal the image editing tools

• Select the “Instant Alpha” tool, which looks like a magic wand (it is contained under the Selection pulldown menu if the image is smaller than certain widths)

• Click and hold on the part of the image you want to turn transparent, and while still holding move the cursor up or down to either select more or less of the image to turn transparent – anything that is red is what will become transparent

• Hit the Delete key, or go to the Edit menu and select “Cut” to remove everything that was highlighted red with the Alpha tool (note: if the original image was a format that does not support transparency, you will be asked to convert the document, choose “Convert” to proceed” as expected
• Repeat as necessary for other parts of the image you want to become transparent

To get the finer details transparent it can help to zoom in and out of the image using Command+Plus and Command+Minus keys.
If the original file was a PNG or GIF and you are satisfied you can just save as usual, but in a lot of cases you may not want to overwrite the original file. Instead, you can save the newly transparent image as a copy by using “Export” or “Save As”.

Exporting the Image as a Transparent PNG or GIF

PNG files are much higher quality than GIF, and for most uses you’ll want to use a transparent PNG, but we’ll cover both anyway.
Saving a Transparent PNG

• Go to File and choose “Export”
• Select “PNG” from the pulldown menu, and check the box next to “Alpha” to insure the image maintains it’s transparency
• Save as usual, maintain the .png file extension

Saving as Transparent GIF

• Go to File and select “Export”, then Option-click on the file formats menu to reveal “GIF” as an option
• Check the box for “Alpha” to preserve the image transparency, then save as usual with a .gif extension

Because you’ll need to perform manual adjustments with the alpha tool, this would not work on a group of files, though you could batch convert them ahead of time to PNG or GIF, then open each individually to make them transparent.
The video below walks through the process of turning an image into a transparent version of itself, including cleaning up areas that didn’t get immediately grabbed by the alpha tool by using zoom.