In this post I will show how you easily can highlight text in an image using the free image editor GIMP.
Open an image file containing text in GIMP. An example is shown below:
Create a new empty layer above it:
Select the new layer. Then use the paintbrush tool with a color of your liking and draw on the text you want to highlight:
Change the mode of the layer containing the drawing from Normal to Darken only:
This is the final result: