Electra has two panels - the original image on the left, and the blurred version + bubble callouts on the right.
To start, drag a local image onto the left panel. Use mouse wheel to zoom in and out, and mouse drag to move the original image around.
Double-click anywhere in the left panel to add a new bubble. Click and drag closer to the center of a bubble to move it around. Click and drag closer to the outline of a bubble to resize it.
Double-click a bubble to add a callout. Drag a callout to change its position. Right-click a callout to delete it or invert its colors.
Click the Save button to save the screenshot with callouts.