Image maps are still a thing in
HTML5 and Gimp can somewhat help with
14.2. ImageMap tool (Filters → Web → ImageMap). More info at Grokking the Gimp,
9.2 Clickable Image Maps. After using the ImageMap tool in Gimp the result is HTML code (not a new image) that you would add to the image code on your web page. This works well on static image areas.
Here's an example of the code generated by Gimp that you would add to your web page.
<img src="imagemap.png" width="512" height="512" border="0" usemap="#map" />
<map name="map">
<!-- #$-:Image map file created by GIMP Image Map plug-in -->
<!-- #$-:GIMP Image Map plug-in by Maurits Rijk -->
<!-- #$-:Please do not edit lines starting with "#$" -->
<!-- #$VERSION:2.3 -->
<!-- #$AUTHOR:Odinbc -->
<area shape="rect" coords="115,210,124,233" alt="Gimp Chat" href="http://gimpchat.com/index.php" />
</map>
usemap="#map" is used in the image tag to link to <map name="map"></map> link coordinate code.
Any 'usemap' name must match the 'map name'.
The <map name="map"></map> code can be placed anywhere between the <body></body> tags.
The 5 lines starting with, <!-- #$-:Image map file created by GIMP Image Map plug-in --> are comment tags.
<area shape="rect" (type rectangle), coords="115,210,124,233" (coordinates of link area), alt="Gimp Chat" (alternative text) and href="http://gimpchat.com/index.php" (the link).
That's the short answer. There are other techniques and responsive image solutions unrelated to Gimp. Is your web site responsive (mobile friendly)?