Hm, everybody is free in how and for what reason images are posted in whatever resolution. However, I personally think that limiting the resolution as a policy is against the spirit of this forum (and probably the spirit of the cc licenses, not their possibilities). If somebody licenses only small images with cc and makes money from the full size pictures is fine, it’s the choice of the original producer of the work. In my opinion, however, one fundamental aspect of the whole FLOSS software movement and the “ecosystem” that grew around (such as cc licenses) is that it allows learning from things others published (e.g. learning from the code of others, or, e.g., the photo editing of others). The PlayRaw threads, e.g., are a perfect example for that.
As said, the images should be downloaded in full size on demand only, not always. This is something that for sure can be solved technically. For the upload traffic, I am sure this does not play a role, and if your connection is slow it is no problem, you can always upload a small file. I only argue against making small image upload the default by policy.
I don’t think uploading small images is of tremendous help here. The moment an image is on the internet you are losing control over how it appears at the user, since you cannot control any of the viewing circumstances (display size, display color management, display brightness, surrounding light, time of day, mood of the person in front of the screen, …).
As a professional photographer (meaning I have to make my income from photography), I would hardly put those pictures I want to sell in low resolution on pixls.us to hope somebody wants to buy a full size version. I think there are better ways to increase the success of my photography job, e.g. by showing my skills on pixls.us (e.g. by posting images that are looking good even in 100% view) and hoping I will get jobs from that. But overall, I do not think pixls.us is a good place for advertisement at all, there are for sure better places for that. For me, pixls.us is rather a place to learn.
Don’t worry, the amount of work for chris does not count anyway.
Unfortunately, 1 of the 2 images that had less than acceptable resolution have been edited with Gimp in a second step. Therefore, I had to cheat a bit and reproduced these edits (fortunately it was not too much, only some cloning out of flares, which I was able to reproduce with darktable 2.5 then ).