I'm creating a login form like the following.
Now I have the color versions of the images but I don't know how to make them black and white. Can you help me?
To make a colored bitmap black and white in Gimp follow these steps:
Adjust Colors > Hue-Saturation to Saturation
Make grey tones black using the Colors > Levels tool by dragging the black point to the right until happy (here I used
150 for black):
Make the white background transparent with Layer > Transparency > Color to Alpha choosing (default) white as alpha color:
Only in case there was no antialiasing from the originals we may also transfrom a colored image to black and white whithout any grey tones using the Colors > Threshold tool but this usually will lead to considerable aliasing artifacts.