5-10 USD one time sounds fair, annual sounds like a big nope.
4.99$ or 9.99$ one time fee or 1.99$ a year feels about right for me. I would prefer the one time fee because I don't expect it getting regular updates(I'm fine with paying again for large version updates. I'm already paying for iStat Menus, Affinity Designer, BetterTouchTools and they all work with this business model). It is a utility that I would use every now and then, so subscription feels wrong.
Annually? Eww.
I feel the same with Gitkraken. There has literally been nothing added to it since the first release that I would pay for, but as a subscription they have to tweek it.
$20 one time is all I need. I think Jetbrains has the best model for this.
There are a bunch of these already. They’re either free or $5-20 one time. Discovery/marketing will be your challenge with this.
There are no alternatives that nailed it. I think if the OP successfully crates something on par with MS Paint it can spread by word of mouth because there's no good answer for the question of "how do you put these two images next to each other and add a text on top of it".
If it is literally exactly the same thing, I would pay $50 one time for sure.