It's very simple to test image with active storage and with the usage of helper fixture_file_upload:

test "should update project" do
  img = fixture_file_upload('files/esempio.jpg', 'image/jpg')
  patch admin_project_url(@project), params: {
    project: {
        name: @project.name,
        body: @project.body,
        images: [img]
    }
  }
  assert_redirected_to admin_project_url(@project)
end