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

To make all white color transparent in GIMP, first, add an alpha channel to your layer by right-clicking the layer and selecting “Add Alpha Channel”. Then, use the Colors > Color to Alpha tool to make the white color transparent, or use the Fuzzy Select (Magic Wand) tool to select the white area and press Delete. Finally, export the image as a PNG to preserve transparency.

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/