convert -gravity South -fill white -undercolor '#00000080' -annotate 9x9-30+105 'S2' extdomains_5.gif extdomains_6.gif

-gravity {South}: placement on the bottom of the image
-fill {white}: white font
-undercolor: background color behind text
-annotate {xaxis rotation x yaxis rotation, +/- xaxis position, +/- yaxis position, text}
	increasing x/y axis rotation -> turns text clockwise
	0x0 -> parallel to x axis
	90x90 -> text rotated clockwise 90 perp to x axis
	180 -> text rotate clockwise 180 parallel to x asis (upside down)