What tree is this?

Three photos beat one: the whole tree for its silhouette, the bark up close, and a branch showing how the leaves are arranged. Leaf shape on its own is ambiguous across a lot of species. Leaf shape plus opposite-versus-alternate branching plus bark texture usually is not.

Sometimes. A ginkgo leaf or a shagbark hickory’s bark is unmistakable. Most trees are not — red and black oak leaves overlap heavily, and young bark on almost anything looks like young bark on anything else. Send what you have; the answer tells you how sure it is.

Reliable to the genus nearly always, and to the species most of the time, when the photos include bark and branching rather than a single leaf. Cultivars and closely related hybrids are the hard cases, and the answer says so instead of guessing.