Hi Mykola,
Try the following, -gravity south -annotate 0 'You text here'
.
-annotate is used exactly to place text in an image but it seems to require the usage of -gravity to indicate where the text will be placed. Then you can use other parameters to set -font, -pointsize, -fill, -stroke, etc.
I've tested it with the command line but it should work the same for the extension.
Cheers