How can I take an image with a white background and make it transparent

"you can use Photofiltre (small, fast, free and portable):

load the image
go to Image > Transparent Color...
choose the color (white), set the desired tolerance with the slider, click OK
go to File > Save as... and save the image as .PNG"

"I haven't seen a solution using Paint.NET here yet. It's an awesome lightweight image editor. To quote Jeff: ""It raises the quality bar,""

Download, install and run Paint.NET
File > Open > Select your image
Click the Magic Wand tool:

Click the white background on your image
Press Delete
File > Save

You can also select the tolerance of the magic wand tool before your use it to get rid of the ""crust"" or whatever you want to call it around your leftover image."

"You can use Paint.net to do this using the plugin Alpha Mask.

Load the image. Select All (Ctrl-A) and Copy (Ctrl-C).
Using the Effects menu, select Alpha Mask
Leave the Mask File box blank and check the following options:

Invert Mask
Mix Alpha
Paste From Clipboard

After running the effect, you can optionally duplicate the layer and merge down to ""darken"" the image.

I know that Paint.net was used in a previous answer, but using the Magic Wand tool, or the Paint Bucket (to fill with transparency), will leave the edges of the image jagged. It won't look good on color backgrounds. Using the Alpha Mask method your image will look crisp on any background."