I know you said you're not comfortable with command line tools, but ImageMagick can do this:
Replace a Specific Color
convert balloon.gif -transparent blue balloon_trans.gif
balloon.gif is the source image,
-transparent specifies that you want a transparent bg,
blue is the color you want to replace, and
balloon_trans.gif is the completed image.
This is under the assumption that your "bright pink" is not part of any of the images. For a little more intelligent background removal (floodfill), check out this tutorial: Masking Simple Backgrounds (floodfill)
This can get a little hairy and some of the options they specify there probably aren't required for what you're trying to do, so as @graphics man suggests you can add
background to the command
If you can tell me what OS you're using and a sample image, I can help you with a script to automate this for many images
Specific to your situation:
First, install ImageMagick. You may need to restart after installation for your PATH variable to be updated (I did)
Once it's installed, fire up a command prompt (Start Menu, type
cmd, hit enter)
You need to change directories to where the images are saved. For your situation, enter the following command:
Now we need to make a folder to save all the images in:
And now, the command to convert all the images:
FOR %G IN (*.png) DO convert "%G" -transparent #FF00FF "batch\%G"
Note: With ImageMagick v7, use
magick instead of
convert, as so
FOR %G IN (*.png) DO magick "%G" -transparent #FF00FF "batch\%G"
If everything went smoothly, the newly saved images should be in the batch folder you created.