Above the fold, meaning the viewer shouldn't have to scroll to see them. This makes the ad space more enticing to advertisers. However, they should also be placed in a way that doesn't push your actual content below the fold, since you don't want to scare away or confuse your readers.
If you have ads above your comic page, then most of the comic page should still be visible without scrolling, which means you have less/no room for a header of your own. Another option is placing skyscraper ads next to your content rather than above it, but you need to be careful with these, as major networks don't tend to carry skyscraper ads, and they're large and can therefore be quite distracting.
If you foresee your comment section being very active (some types of comics are less likely to than others), then having ads below your page but above the comments can work, even though it would likely place the ads below the fold.