I hate transparency in images. More accurately, I hate having to make it happen – I think it is a fine concept.

Turns out that Preview on the Mac handles this. From http://scripting.com/2014/11/27/aHiddenMacPaintProgram.html …

• Copy an image to the clipboard.
• Launch Preview.app.
• Choose New from Clipboard from the File menu.
• When the image window opens, click on the toolbox icon to enable the editing features.

Things you can do:

• Resize/scale an image.
• Make the background transparent.
• Draw an arrow on the image.
• Save as either GIF, JPG or PNG.

Step-by-step with pictures is at http://osxdaily.com/2013/01/27/make-a-transparent-image-png-or-gif-easily-with-preview-for-mac-os-x/