Try the Spawn New Window action. It supports all manner of sizing and window positioning for the windows it opens, and it includes a Close Button action. Simply apply that to the big picture, and a click will close the window and return to the gallery. This is one of my favorite design patterns for child windows.
Thank you for your help. But, if I understand correctly, I have to create a page for every picture used in the website. Link to image is such an easy action, select the little pic, apply the action, search for the bigger size picture on your harddrive, and ready. I know, it links to a picture, and not to a html file.
Anyway, thanks for the help.