Photopea is a great in browser free Photoshop knockoff with basic functionality, really does have its moments, it might do the trick for your needs. Affinity has a sale right now which would make it a worthy contender, though, I must say it has a few features that are superior to Adobe and it has some other features which ... are not. And their staff have no intention of ever improving workflow for anyone who doesn't work exactly like they do. For example, I would like to be able to copy and paste a layer style. It is possible to SAVE a layer style and reapply it later. But it's not simply possible to hotkey "copy the layer FX" to a keyboard shortcut, and for some weird reason it only has "paste FX" in the dropdown menu and only lets you do it once. The staff don't give a crap though because tHeRe's A wAy To Do It. Right, but with several more clicks than I want.

It's good for the price. If you're only planning to do this once, try Photopea or Gimp first.