Yes and no. Are you saying that ALL future competitions will follow this format? If not, ignore the following. If so, we're risking making competition a "Caption This:" affair.
Competitive-wise, there's no point of having a picture at all if there can't be big differences. If X+Y > Z+Y, then X > Z where X and Z are card ideas, and Y is the predefined art.
Aesthetically, yes, Zanz can use fan-art. But he's only getting one image from each competition. If he doesn't like it, having the image was pointless. While it saves work for him, the picture partly defines and limits the idea. Would this be a fair trade-off?
For example, let's imagine that we used this format for the weapons competition. The most flexible stock picture would be, off the top of my head, a glowing sword, since you can slap a lot of ideas to it. Perhaps with various tints ala the fountains in the new competition. But an ability such as the (currently) 2nd Place Boomerang might have not shown up, as a throwing a sword is a good way to disarm yourself. Perhaps an enchanted returning sword, maybe, but the idea would be less likely to have come up as people are thinking "Sword".
Let's use another example. Let's say we have a creature competition. Is the stock picture creature big? Small? Pretty? Ugly? Humanoid? Quadruped? Etc. Making the creature generic enough to suit all possible ideas would likely make it unusable as an actual card. Permanents share the same problem, as the variety of objects is almost limitless. Spells you might be able to get away with this.
In short, a picture and the corresponding idea are tied together. If one is constrained, the other is as well. Is this worth the benefit of having a card ready-to-go?
And again, if you're just using this to supplement the existing competition types, then the above doesn't apply. Though if you want to avoid image bias, text-only may be the only way to go.
