You can use Boxoft Batch Photo Resizer to rename a great number of images at a time. it also lets you to resize, rotate, etc pictures. especially you can convert original to other formats: tif, bmp, gif, jpg, ico, j2k, png,tga, pcx, etc.
More detail follow steps below:
Press "Add" or "Add Folder" to add photos;
On the bottom menu, write your text stamp down on the down right area;