impress images save to pdf

Hi this is a question I have not been to able to find an answer to.

I often make images using Impress for scientific presentations. These are simple graphs or explanatory figures. I could make these images in Gimp or Inkscape, but for some reason I find making such images in Impress much easier.

My question is how to export these images. I used to do a screengrab and then crop with GIMP and save as PNG. However, the images in Impress are in vector format, and so I would ideally like to save them directly to something like PDF. How to do this? Again, I am not interested in simple exporting an entire slide to a PDF file. I just want the image. I am using open office 4.2.8.2 in Ubuntu 14.04.