in photoshop? i use the "batch" command along with "actions" command in photoshop. I'm sure there are other ways to do it, but this is how I do it.

First I make a separate file just to creat the watermark and save it as a psd.
Then I make the action for it.
The action stuff is under a tab on the right hand side, 3rd box with the "history" tab. I make a separate folder with my custom actions then hit the "creat new action" button next to the button that looks like a trash can. then I start my magic. I open up a test photo or the first photo from the batch that i want to work on. (do any processing to the photo first like levels/curves/whitebalance/cropping/etc)
Then I hit the "record" button that looks like a circle on the bottom of the actions window. now, while it is recording my every move, I open the sig file I made earlier, copy all (easier to have the file size to what you want with a blank background layer), close the sig file, hit "image > image size...", resize image to 800px width, and paste the stuff in the sig file onto the photo you are working on.
Then I place/size the sig where I would want it to be (for me on the bottom right), set the opacity (if needed), and then this could be the end of the action if you want to keep it for one-by-one photo editing then finish off with a quick watermark. (then you can hit the square "stop" button in the bottom of the actions window.
But since you are going to do a number of photos in batch, you don't want to stop the actions recording just yet.
Next step is to save the file in a folder. I like to "save as optimized for the web" feature to keep the file sizes low for posting on the interw3b. So I hit "File > save for the web," make sure to put jpg and slide the % to 50% or 75% or whatever you want then save the file in a folder you want to put them in. after it saves, hit the close button on the photo you are working on and cancel button when it asks if you want to save what you are doing (assuming you don't want to save anything you did to the original photo). once that is done, you can then finally hit the stop button on the actions window.
Next you want to do the batch. First make sure you have a folder with the photos you ONLY want to do the batch work on. then you hit the "File > Automate > batch..."
then you set the settings for the batch command in the next window.
Set is the folder where your custom action you have just made above.
Action is well, the name of the action you just made.
Source should be on "folder"
Hit the "choose" and select the folder where all the photos you plan to batch is.
then the only box i checked under is "Suppress file open options dialogs"
Destination should be "folder"
Hit the "choose" and select where you want the finished watermarked/resized photos to be.
Check "override action "save as" commands"
file naming is up to you.
then when all of that is set, hit the OK and go get a cup of coffee or beer while photoshop does the rest.
at the end it might give you and error saying it can't process the thumbnail file then just ignore it since you are finished.

thats all there is to it. enjoy