Trying to use node comments for a "picture of the month" section.

Hi, everybody. I've been trying to solve this the whole day with no success.
So I'm asking somebody for help, english is not my first language, please excuse my writing.

I'm trying to implement some sort of a competition section in my site. Like a "picture of the month" section.
The site is babies-related and the idea is that registered mothers visiting the section can upload their baby's images for being voted online.
There should be one competition per month aprox. and by the end of it the most voted picture will be shown at the front page and a new competition will be held, closing the prior one.

So I thought about the following solution:

* Using "CCK" module I could create one node type (competition).
* Using "node comments" module then I would use a CCK node type that allows to upload an image via one of the "imagefield" or "image" modules, the image will be uploaded as a comment.
* Using the "commments limit" module I could stop allowing posting pictures 30 days after the competition opened (competition node being published).
* So, creating a new competition node would open a new competition.

Teoretically it should work, but it seems I cannot make the "node comments" module to work. I created both node types and asigned the node type "comment" as comment of the "competition" node type.
I also used CCK to add an imagefield to the "comment" node type, to check if it would appear on the comment form.
I made all the required steps, but when I try to add a comment the form that appears corresponds to a core comment.
In fact, the comment is stored as a core comment and when you display the full node to see its comments they don't appear at all.
For showing them I should run the "nodecomments conversion tool" to convert them.
Once they're converted to nodes I can edit them and add the image without problems and, once the comment is saved, then the image appears at the node's full view.
It seems the core comment form is appearing instead of the "node comments" module form.

Do you guys have any idea of what should be the problem?
I'll really appreciate any advice you could give.
Thanks a lot in advance.