In the question for the weekly image contest, it
says said [n.b. the interpretive statement has been removed; see below].
The submitted image must conform to this site's content license, cc-wiki with attribution required. This will only apply to the low-resolution version provided on the site directly, not to the same photograph in other formats or to linked images, but if you're not comfortable with that, this isn't the contest for you.
I don't see how this can be possible. Although I can't make a solid case with reference to the wording of the license alone, consider the following sections from the creative commons website. The second section is linked to from the first section, so the logical flow goes directly from the first part to the second. Emphasis is mine.
You may license your copyright or distribute your work under more than one set of terms. For example, you may publish a photograph on your website, but only distribute high-resolution copies to people who have paid for access. This is a practice CC supports. However, if the low-resolution and high-resolution copies are the same work under applicable copyright law [see next section], permission under a CC license is not limited to a particular copy, and someone who receives a copy in high resolution may use it under the terms of the CC license applied to the low-resolution copy.
Note that, although CC strongly discourages the practice, CC cannot prevent licensors from attempting to impose restrictions through separate agreements on uses the license otherwise would allow. In that case, licensees may be contractually restricted from using the high-resolution copy, for example, even if the licensor has placed a CC license on the low-resolution copy.
As with most copyright questions, it will depend on applicable law. Generally, to be different works under copyright law, there must be expressive or original choices made that make one work a separate and distinct work from another. The determination depends on the standards for copyright in the relevant jurisdiction.
Under U.S. copyright law, for example, mechanical reproduction of a work into a different format is unlikely to create a separate, new work. Consequently, digitally enhancing or changing the format of a work absent some originality, such as expressive choices made in the enhancement or encoding, will not likely create a separate work for copyright purposes. The creative bar is low, but it is not non-existent. Accordingly, in some jurisdictions releasing a photograph under a CC license will give the public permission to reuse the photograph in a different resolution.
From these two excerpts, I would surmise that any high resolution copies must also be licensed under the terms of the cc license as the uploaded low resolution copies, since there are no "separate agreements" imposed and since I don't think any court in any jurisdiction in the world would consider downsizing an image to constitute "expressive or original choices made that make one work a separate and distinct work from another". The only choices made would be interpolation algorithm (most people probably just used the default for whatever program they were using), output size (though this is dictated by the competition guidelines) and jpeg quality. So basically no creativity whatsoever.