I discovered the download limit on WinCustomize today.
It seems you are allowed a free registered account (as opposed to an anonymous guest) if you recently purchased a Stardock product, even through the Humble Bundle. I just registered and I didn't think it was entirely obvious, thinking if I owned a Stardock product, it would auto-generate a user account. I registered through WinCustomize using my Stardock account info, confirmed via email and THEN received a notice that I was now registered on all Stardock websites.
So now as a registered free user, it no longer looks like I have a download limit, BUT.... my download speed is much, much slower. I randomly picked a 93 MB background and it is taking me about 35 minutes to download, when ordinarily it should take less than a minute (really, about 10-15 seconds). I'm not complaining, free is free and we get what we pay for.
I also searched and found a few other websites that have free Dream desktop galleries and so far they look like they kind of suck. I haven't browsed too much, but right off the bat the desktop images they feature are edge-to-edge busy, with lots of colors and things happening. For me, busy backgrounds are distracting, headache inducing and icon obscuring. I'm going to go take a look at some websites that are more oriented to art and design and see what, if anything, they offer there.