The first thing to notice, as pointed out in the comments, is that time increases going up. So if you are more familiar with viewing Feynman diagrams where time increases to the right, this problem is easily solved: just rotate the diagram by 90 degrees when you are interpreting it.
If the problem is that you're not all that familiar with matter lines in Feynman diagrams in general, here's the rule:
- If the arrow points in the direction of increasing time, it's a matter particle (in this case, an electron).
- If the arrow points in the direction of decreasing time, it's an antimatter particle (in this case, a positron).
That's it. The arrow on the line doesn't indicate the direction in which anything is actually moving.* It's simply a convention to show whether the particle is matter or antimatter.
This rule is convenient because, if the diagram is correctly drawn, you can trace along a lepton (or quark) line from one endpoint to another, always going with the arrows.
Note that sometimes you'll encounter an arrow which points in the spatial direction, perpendicular to the time axis, such as the horizontal line segment in the middle of this diagram. Don't take that segment too literally. It doesn't mean that the electron actually moves in a spacelike direction. It's just "code" for whatever (leading order) processes actually do occur in which the electron and positron annihilate with each other while emitting two photons in the process.
*Actually, it does indicate the orientation of the current of the corresponding field, but for purposes of interpreting the diagram, I'd suggest you just consider that a technical detail of the QFT calculations and not worry about it.