Just so you know how to do it. There isn't a CTRL + A (Select All) for layers. But you can SHIFT select layers. So, select the bottom layer then scroll to the top and SHIFT + CLICK on the top layer to select all of the layers in between.
If you were just loading 600 images, changing the blend mode to lighten, I would do this with a script.
Yes, you can just drag all of the images into the layer palette from explorer and have it open all of the images as layers into your current image. And you can then just select all of them to change the blend mode and finally flatten the whole image. But depending on the size of your images and the resources of your computer it's probably not the fastest thing around to do that. My script would just open each image in a folder as a new layer, change the blend mode and merge down 600 times. Still slow, but probably a lot faster in the long run as it uses less memory to do all of the work.
Irrelevant now that you are using a software dedicated to this requirement.
