It needs to be 24x24 other way around, Iris wont allow you to use. Probably it have the constrain proportions, unlock that option in the window where you change size. That way you should be allowed to change proportions freely.

but If I want to change my image size it can't be exactly 24x24 in photoshop what should I do?

This may help. There are a few guides for Emblem making in the guide section.
http://noscrubs.net/discussions/recent-discussions/how-to-make-a-guild-mark-transparent-with-photoshop