Pop up window and text

Hey everyone! I'm making up an image gallery that has thumbnails that click open a pop up window with the bigger image inside. I also want to put text in that pop up window but different text for every image. How would I go about doing this? I can get text to show, but it stays the same for every image instead of changing everytime a different image is opened up. I've been trying to figure it out and I'm getting nowhere.

Thanks for the help in advance!