If a picture helps, use it. I know people who turn their work rooms into something out “A Beautiful Mind” with pictures and tape connecting them. We all work differently.
On the darkness of many flash fiction stories. Yes. That’s the usual. I bought a collection of flash fiction stories and found that 90% dealt with death. (I think the Woman’s World ones are mostly positive, so those might be worth exploring.) If it a form you don’t like fine.
You might consider looking for content inspiration in collections of one act or very short plays. I’ve found those have a lot more variety in themes and genres than most flash fictions.
By the way, if you choose to write a positive flash fiction, it will immediately set yours apart. And likely appeal to an editor buried in dark tales.