Hi!

I admit I had to look up 'heuristics', and even now I'm not quite sure how it applies, but I can give you some general advice:

For the actual cover images, I use Gimp which is basically a free Photoshop. Yous shouldn't need anything else.

If design is your speciality, you'll probably have some ideas of how to incorporate your work into a striking design that will make people want to click on it. If it isn't then check out the competition- look at the thumbnails of pro asset packs and analyse what you think works or doesn't about them, and apply this to your own stuff. You should aim to create something that you would want to click on if you saw it.

Know the dimensions before you start (cover images, thumbnails and banners are often different sizes on different sites). The last thing you want to do is complete your design and find you've got to change everything again, or you've made it too small and it looks blurry on itchio.

Always use layers. If you put different elements of your work on different layers, then if it all goes tits up (which it often does), you won't have to start from scratch. It also makes it easier to go back and edit.

Generally, a heuristic is an approach or method that may not guarantee 100% efficiency or exactness in its results, but can provide a reasonably accurate or close answer within a specific domain of tasks. For instance, in a platformer game, it is not necessary to simulate every aspect of physics. Instead, you can simulate certain parts of physics or create a simplified representation using a predefined table (similar to how it's done in Mario games, if I recall correctly). There are situations where an exact value is not required, and as long as the error remains below a certain threshold, there is no need to be overly concerned.