Impedance matching means that one side impedance equals or joins another impedance with a power transfer without reflection.
Your picture 1 shows a line with 50 Ohms facing two parallel 50-Ohm lines: this is a mismatch as the impedances on the left and to the right of the junction are not equal: one is 50 Ohms, the other 25 Ohms.
Fig.2 shows 50 Ohms on the left and 50 Ohms on the right , therefore this junction is matched.
Instead of lines you can consider source and load impedances.