You have to be excellent at graphics for starters. Post a lot (quality posts, not spam), report spammy threads in the graphics sections and just generally make yourself better known in the community.
At the moment, there are two gfx mods so there won't really be much need for another any time soon. Graphics section just doesn't need much attention or moderation.
I was a gfx mod for a few months start of this year but had to quit due to irl circumstances. It's not an easy job and you're expected to file lots of reports, plus help go through scam reports for other sections.