To expand beyond the restrictions listed above, Avatar/Sig infractions are only worth 4 points and expire in a month (vs. normal infractions worth 5+ points and expire in 3 months). Bans don't start triggering until 15 points, which means it's extremely difficult to be banned from signature infractions, so it's not really ridiculous or anything.
Also, your particular signature was removed because it violated the filesize restriction. If you need any help determining an image's filesize before changing your sig, feel free to PM any moderator, and we're more than happy to help with that.
Sig infractions are very different than "regular" infractions. The points from them last only a month, and they can never ban you unless you get multiple (since bans happen at intervals of 5 points starting at 15, and sig infractions are only 4).
The point of a sig infraction is to make a dated, mod-visible note that we've told you about it, so that we know if if someone just made a mistake or is regularly breaking the rules intentionally.
We don't really have any other way to make this kind of note in the forum software, so we use the infraction system, but you'll never be looked down on or treated differently because of it. Many of the mods have even gotten sig infractions themselves in the past before becoming mods.