is there a custom field that will post nothing…

Is there a way to use the custom field feature so that it won’t post the colon automatically?
Basically, here’s what I’m trying to do. Occasionally, I like to add junk into my posts, like an image/image tag or whatever. The thing is, I am going to have guest bloggers come in and I want to give them an easy, standard (and as code-free a) way to add an image tag.
Am I making any sense? If anyone can help me, I’d appreciate it.