The first picture
The two triangles are proved by SSS (SIDE-SIDE-SIDE)
Reason: As you can see, there are marks on the sides of the triangle (three line marks and two line marks). Those marks represent congruence.
The second picture
The two triangles are congruent from Angle-Angle-Side. Try to picture the first triangle by rotating it the same way as the second triangle. You can see the marks and angles are the same.