The Hungarian national contest prohibited watermarks for images last year (and personally I don't like pictures with watermark).
In Wikimedia Commons:
* https://commons.wikimedia.org/wiki/Commons:Watermarks#Reasons_not_to_upload_watermarked_images
* https://commons.wikimedia.org/wiki/Commons:Image_guidelines#Image_page_requirements

Samat

On Fri, Sep 7, 2012 at 7:38 AM, Nicu Buculei <nicubunu@gmail.com> wrote:
On 09/07/2012 12:35 AM, Андрій Бондаренко wrote:
Hi everyone,

Recently one of our participant asked me - could I upload photos with
watermarks? He argues that he loose original versions (without
watermarks) and  their removing demands to much time. What should I
answer him? Are photos with watermarks (as theese
<http://toolserver.org/~magnus/catscan_rewrite.php?language=commons&project=wikimedia&categories=Images_with_watermarks&negcats=Images+from+Wiki+Loves+Monuments+2012&ns[6]=1&ext_image_data=1&doit=1>)
allowed?

I had a look at a few random images there and I can identify a few cases:
- real watermarks (didn't find any in the examples) are when a big watermark covers a large and important part of the image, making it unusable (think at the preview images from stock photography sites). those CAN'T be allowed;
- signatures, small watermarks in an unobtrusive part of the image (most of the time in a corner). I allow those but discourage them by explaining the uploaders that grace to our free license, anyone is allowed to remove them, so is useless;
- some photos have the date watermarked in a corner, this probably happened most of the time in-camera and are unintentional. I feel them annoying but harmless;
- i saw a few cases the name of an educational institution there. For those I would ask the uploader if the image is really free (it may be an internal policy to watermark everything at it may forbid derivatives).

So in conclusion I do not like watermarks, would alow some, would forbid some, would question further some. Case by case.