just like title.I have not found the way to do that in the "ShowDemoWindow"

Before drawing the text, do ImGui::SetCursorPos({ImGui::GetCursorPos().x, ImGui::GetCursorPos().y + (yourImageHeight - ImGui::GetFont()->FontSize) / 2})
Without C++11 you can ImGui::SetCursorPos(ImVec2(ImGui::GetCursorPos().x, ImGui::GetCursorPos().y + (yourImageHeight - ImGui::GetFont()->FontSize) / 2))
Does my previous code solve your problem? If so, close this please.
Yes, Thank you.
Most helpful comment
Without C++11 you can
ImGui::SetCursorPos(ImVec2(ImGui::GetCursorPos().x, ImGui::GetCursorPos().y + (yourImageHeight - ImGui::GetFont()->FontSize) / 2))