Been there, done that.
This one puts a logo.gif onto the image.
convert -flatten earth.png logo.gif earth1.png
This one puts a logo.gif on different places of the image.
composite -gravity southeast logo.gif earth.png earth1.png
If you want to put text for a logo, check out my other notes.
You will need to play with size and position as I was using a small image.
convert xc:white -resize 100x20! blank.jpg | \
convert -append cows.jpg blank.jpg tmp.jpg | \
convert tmp.jpg -gravity south -font helvetica -pointsize 15 \
-fill blue -draw "text -20+0 Cockatoo" tux1.jpg
convert xc:white -resize 100x20! blank.png | \
convert -append tux.png blank.png tmp.png | \
convert tmp.png -gravity south -font helvetica -pointsize 15 \
-fill red -draw "text -20+0 'Not Now'" tux2.png
convert xc:white -resize 330x20! blank.png | \
convert -append cows.png blank.png tmp.png | \
convert tmp.png -gravity south -font helvetica -pointsize 20 \
-fill blue -draw "text +120-3 Cockatoo" tux1.png
convert tux.png -gravity center -font helvetica -fill blue \
-draw "text 0+5 Cockatoo" tux3.png
convert tux.png -font Helvetica -pointsize 15 -fill red \
-gravity center -draw "text 0+0 'Not Now'" tux4.png
convert cows.png -font Helvetica -pointsize 25 -fill white \
-gravity northwest -draw "text 0-10 'Not Now'" tux4.png