Most of the common photo editing programs, e.g. PhotoShop, can add a watermark. Irfanview, a very basic editor & free program, can add user-defined text to a photo.
I use Fast stone resizer - it will allow you t overlay a transparent line of text to your pictures in a position of your coice. It can handle batches of photos really easily and quickly too. Best of all it's free, Just Google the name.
Have a look at my site if you want to see how I use it.