If you have load the image in AppGameKit via agk::LoadImage(YOUR_IMAGE_ID, "your_image.png") then you can use
ImGui::Image((void*)agk::GetImagePtr(YOUR_IMAGE_ID), ImVec2(agk::GetImageWidth(YOUR_IMAGE_ID), agk::GetImageHeight(YOUR_IMAGE_ID)));
But if you don't want to bother the AppGameKit Image Manager and the image should really only be rendered with ImGui you can also load the image directly via cImage.
cImage* yourImage = new cImage("your_image.png");
then the ImGui line would look like this.
ImGui::Image((void*)yourImage, ImVec2(yourImage->GetWidth(), yourImage->GetHeight()));