Can I have a header page leading into my primary index page?

Did you install WP into your root folder or into a sub-folder?

If you installed WP in a sub-folder it is easy. Just put your photo page in the root folder and call it index.htm You will need to check your root folder .htaccess file to make sure index.htm is not being redirected to the location of your WP installation. If it is, remove that redirection.

If you installed WP in the root folder, you will want to add this line into your root folder .htaccess file
DirectoryIndex index.html index.htm index.php

That is a directive to the Apache webserver used when someone types only https://domain.com in their browser – no specific file being requested. Apache goes down the list in order and serves the first file in the list it finds. As index.php is last in the list, index.html or index.htm, if they exist, will be served instead of index.php

The blog is in a subdirectory, but the blog index file is in the root. Will this fix still apply since I can’t put the photo page in the root and have it also named index? Can there be an index and an index2? Or another name that supdercedes index?

yes, the files can have any name. I just used index in my examples since that is the typical name used.

Apache directive doesn’t care what file names are in the list. It just goes down the list and serves up the first file in the list it finds, ignoring the rest.

I just checked and it appears you can also list files in other folders. In other words, you should be able to say
DirectoryIndex index.htm photos.htm /blog/index.php /photos/index.htm

For anyone interested, the Apache page is here
https://httpd.apache.org/docs/1.3/mod/mod_dir.html