Guild Emblem Ideas (EoD updated! Contains spoilers.) : r/Guildwars2 Guild Emblem Pack (2862) - Other - rAthena
Create your logo at 32x32 or 64x64 first to get the design right. Then, use an image resizer to shrink it down to 16x16 . This helps you visualize how much detail you lose before you start pixel-pushing.
Several tools and software can help you create a 16x16 guild emblem: