I would recommend starting with this guide for doing classification, not object detection:
https://kiosk-dot-codelabs-site.appspot.com/codelabs/tensorflow-for-poets/#0
Classification is for one unique tag for one picture (99% square, 1%circle). Object Detection is for classification of several objects within the picture (x_min=3,y_min=8,x_max=20,y_max30, 99% square). Your case looks more like a classification problem.
You don't need the full Docker installation as in the guide.
If you have Python 3.6 on your system, you can just do:
pip install tensorflow
And then jump to "4. Retrieving the images"
I had to try it out myself, so I downloaded the first 100 pictures of squares and circles from Google with the add-on "fatkun batch download image" from Chrome Web Store.
On my first 10 tests I get accuracy between 92,0% (0.992..) and 99,58%. If your examples are more uniform than a lot of different pictures from Google, you will probably get better results.