I got this as a comment from Joe Nicora over on my Easy image insertion with HTMLArea post. I think it makes more sense here on the support site, so I'll answer it here.

His question was:

How would you go about Preventing Image Resize?

Well, the dirty little secret is that HTMLArea actually pastes in the full size image by default -- you can't currently choose one of the thumbnail sizes. It's in the works!

Also, in case this was a question about uploading, older versions of the image module would actually resize upwards to the thumbnail size, which looked pretty terrible. The new version doesn't do that.

Your site might be configured to only accept a certain maximum image size, in which case uploads are automatically down sized to that maximum. If you want full-size images all the time, increase or remove the maximum image size.