Tasks

Remove a certain color

• add alpha channel if missing
• use the “select by color tool”
• click on the color to remove
• Edit/Clear
• Edit/Fill with color

Position something precisely

Use the grid:

• Image / Configure Grid
• View / Show Grid

https://docs.gimp.org/en/gimp-image-configure-grid.html

or guides

Create a CAPTCHA

Using the cage transform tool…

https://www.instructables.com/How-to-make-a-CAPTCHA-image-in-GIMP/