Question

Is there a way to "write" on images inserted on an answer?

Answer 1

Not directly, but you could edit the image before you upload it.

Explanation:

If I want to add text to an image, then I would recommend using a free vector graphics application called Inkscape - available at https://inkscape.org . This is available for Windows and Mac OS - though it is a little more cumbersome to run on Mac OS.

Inkscape is also very useful for drawing diagrams of all kinds.

To add text to an image, you would import the image into Inkscape, use the text tool to add text in whatever font and colour you like, then export your drawing as an image. It is easy to change the orientation of text too, to label vertical axes, etc.

Answer 2

Another free package you could use is GIMP. You will need to be superb at mouse control for writing or better still, use a tablet.
The advantage of Gimp is that you can use layers.