Many designers come here to show off their sketches. They don't have to show off the entire design, and designers will take precautions, such as presenting images from a raked angle - it allows you to see the design while making it difficult to produce an accurate flat image - or incorporating watermarks.
Does that guarantee that someone won't steal your idea? No, not at all. But you can prove prior art easily enough by printing out the page here where your sketches appear, complete with timestamps. Honestly, though, in most cases people won't bother - there's not enough return on the effort it would take.
Only one have I ever heard of a custom deck being forged - the Curator deck was published in Russia by a company with no rights to it before the US release. The only reason they were able to do it was that at the time, the designer, Emmanuel Jose, released perfectly flat, squared, unwatermarked images of the entire deck online - he actually didn't intent to release it as a deck when he created it; it was an art project, with each card being several inches high and wide. These days, he takes measures to prevent unauthorized copying, and he doesn't have those hassles anymore. The forged decks are still out there, but they have the wrong backs on them and are on terrible pasteboard stock - no one in their right mind would prefer it over the actual deck, which is still available today.