I'm trying to add a logo to over 100 images. I have all of the images loaded into a PSD, and the logo as the very first (top) layer, locked into place. Is there an easy way to export layers to files and include the top, locked, logo?
I wouldn't do it that way, not that I am saying that you couldn't do it (*see note below), just that I wouldn't.
The easiest way to apply logos to an image/watermarking would be to record an Action in Photoshop, and apply the action to a folder containing all the images, using File > Automate > Batch. The idea here is not to export PSDs with layers, but to export the images in a format such as JPEG/PNG for sharing on the web, etc. Here's how I would do it.
Open one of your images.
Open the Actions panel, click on the menu icon there, and choose the option that says "Allow Tool Recording"
In the actions panel in Photoshop, create a new action, and name it so you can find it easily, and hit the Record button.
Place, resize, and position the logo where you want it.
Using File > Export > Save for Web (Legacy), save the image in whatever format you want to a new folder.
In the Actions panel, hit the Stop button to stop recording the action.
Close everything that is open, and save nothing.
Click File > Automate > Batch
In the "Action" dropdown, choose the Action you just recorded.
Choose the source and destination folders. The source folder is the folder containing all your images. The destination is the folder where you want the images to be output.
In the options, uncheck "Override Action Open Commands" and uncheck "Override Action Save Commands".
Warning: make sure you don't overwrite any of your original image files, because once you do that, you won't be able to undo it. It's safer to work on a copy of the folder containing the images. When you need to supply the images without the logo/watermark, you should already have the originals saved without the watermark.
*Note: I suppose you could modify the Action recording to your requirements (Step 5: to save a PSD instead) if you really do need a PSD with layers, although personally I can't see the point.
You can create layer comps for each layer and then export each layer comp to individual files. It does mean manually turning on the visibility for each picture layer and then creating a layer comp. this is the kind of repetitive task that would have me pulling my hair out.
A much simpler solution, if you have a copy of Lightroom, is to import all 100 images into Lightroom, then use the logo to create a watermark and then export all of the images with that watermark.
I had to do this exact task a few years ago, and it is what led me to begin using Lightroom in the first place.