How does he pardon himself without being president? Even so, aside from public relations, why would he have to be pardoned?
1) It might be argued that "high crimes and misdemeanors" as a qualification for impeachment allows Congress to instantenously impeach a felon who becomes President. In this case though the election is still valid and succession flows to the Vice-President. I doubt the guy could be elected and then impeached, but Congress could make the threat.
2) In the case of Debs being elected from prison, he's still a felon when he becomes President. However, Presidents have usually frowned on pardoning themselves, but nothing prevenets them from doing so. He needs to pardoned so that it's not a crime for him to leave prison and live in the White House.
3) The one catch is that Presidents must be citizens of the US. For much of the 19th century and early 20th, felons were sometimes not considered as citizens (IIRC). Hence, I think Rutherford B. Hayes once addressed a crowd of prisoners at a pentitentiary as "my fellow citizens"; it was considered a gaffe at the time, since the prisoners weren't citizens. Thus, a felon could under certain circumstances be considered to be disqualified on these grounds. Especially before the 14th Amendment.