Hello I just have a test with a tool for seo and it shows me a problem the title H2 tag is abscente and this hurts your SEO in search engines. I want to know why the H2 tag is not integrated in zenphoto.

There is nothing integrated in Zenphoto itself. What headline structure is used is a matter of the theme you are using. Which you didn't tell btw. Official theme do use the h2 tag and generally the structure h1 (site title), h2 (pagetitle/breadcrumb) etc. (please no debate about if the site title should be h1 or not)

So, each theme auther can do as he likes. If you have other requirements you can always create your own theme.

Ok Tanks you for your answer