Hello Patricia,
Thanks for your question about adding a fixed header and setting up a gallery. You can add CSS (if you are familiar with it) to make any of the headers “sticky” so that it stays on top. It would be easier to simply use a plugin such as this one. You can also create a gallery and link it to a post that gives a bigger version of the picture. When you lick on Add media, you will see an option to add a gallery there. In order to create the linked posts, you will need to first create those posts with the image, and then obtain the URL from the finished post. You will need to have 2 images – one for the post and one for the gallery. The image for the gallery can be edited in WordPress. At the bottom of the edit screen for the image, you will be able to put the link to your detailed page on the image. This way, when people click on the image, it will jump to the detailed post. Make sure that you also create a link to go BACK to the gallery. You can do this by adding the URL of the published gallery to text within the post that contains your gallery.
I hope that helps to answer your question! If you require further assistance please let us know!