Open the image in GraphicConverter
Look at the window name; the "mode" is usually indicated (could be "RGB", "Grayscale", or "CMYK"), and it will also indicate whether or not the image has an ALPHA channel (the transparency channel). If there is no ALPHA, create one by choosing the menu item: Picture -> Alpha Channel -> Add Alpha/Mask Channel
Choose the "Fill" tool (looks like a bucket of paint spilling); double-click on it to add some reasonable tolerance (50, 80, 90 something like that).
Then click on the "Foreground" color; go the "named" colors and choose "Magenta" (I'm assuming your image doesn't have Magenta as a color)
Click on the background parts of your image, turning them all to Magenta
Double-click on the "Transparency" tool; choose "select" to choose a color to turn to "Transparent", and choose "Magenta" and check the box that says "Make color transparent". Then click okay, and use the Transparent tool to click on the Magenta somewhere in the image.
Choose "Save as" and save your image as a .TIFF or a .GIF (some format that allows alpha channels; note that JPEG does NOT allow alpha channels).