This tool allows you to add labels to an image. The image can then be downloaded again with all the labels included. Nothing fancy. Just semi-transparent boxes with text.
All labels are defined via line in a text box using the following format
<alignment><x-coordinate>,<y-coordinate> <text>. You can click into the image get the clicked position
added to the current line.
Just try it: https://splitbrain.github.io/imagelabeler/
Everything is done in browser. There's no image transferred to any server.