I used to use monosnap for annotating screenshots, but it went subscription-only (or maybe it was always subscription-based, but a previously-free-tier core feature I needed for my workflow, the s3 integration, went into a subscription tier).

The best alternative I found is shottr ( https://shottr.cc ), and I cobbled together a thing that scratches my itch: https://github.com/philsnow/shots-filed