The Plan Image fieldtype, just like the Image fieldtype, returns the URL to the image it's displaying when using the raw substitution. It does not return the plan id. This means your condition should probably just REGEXP for the image filename and should accomplish the same thing. If you want to show a plan image for users without a subscription I'd simply check that the plan image field is empty. Example as follows.
[cb:if cb_userplan!=""][value][cb:else]<img src="FREE_USER_IMAGE_PATH" />[/cb:else][/cb:if]
This would be placed in the profile value layout parameter under Parameters > Layout of your plan image field. See the substitution tutorial below for our new elseif and else condition usages. Note you need latest CB build for elseif and else usages to work.
www.joomlapolis.com/documentation/279-community-builder/tutorials/18353-using-substitutions-throughout-cb
www.joomlapolis.com/blog/kyle/18708-substitution-power-up