To crop images in the Uploadcare panel or dialog, you need to configure a cropping step for your widget:
For dialog: in the dialog constructor, add crop setting.
var dialog = uploadcare.openDialog(null, {
crop: "300x200"
});
For panel: add the crop setting to the uploadcare.openPanel call as the last parameter.
var panel = uploadcare.openPanel(placeholder, null, {
crop: "300x200"
});