Besides annotating screenshots, 1Capture also supports annotating local images. You can open local images in three ways:
  1. From the top menu bar, choose 1Capture > Open Local Image
  2. Drag a local image onto the 1Capture icon in the top menu bar
  3. In Finder, select an image, right-click > Open With > 1Capture
After that, annotating local images work just like annotating screenshots.