Well here's how to solve that.
We'll take this post as an example. We originally found the infographic via a Jeremiah Owyang share. However in Jeremiah's share, you can't see the infographic without leaving Plus. We wanted to keep the image in Google+ so people can view it directly in the Lightbox.
If we had tried to post the image directly into our Share box, it would be too small for people to see, so what to do?
Step 1 - get your tools
Well first you need a tool like Screen Capture. This useful Chrome extension allows you to save content directly from your browser.
Step 2 - Chop Chop!
On the infographic page you then simply "chop up" the infographic into smaller pieces and save them as individual images. You do this by selecting "Capture Screen Region" and choosing the area you want.
Step 3 - Save the image
You can then save the parts of the infographic as images to your computer. Name them something easy like "Part1" "Part2" etc.
Step 4 - Create an album
Back to Google+ (finally!) and open the share box. Click on "Add Photos" and "Create an album" (call it whatever you want. The name of the infographic is probably best.)
Add your images in order.
Step 5 - Post!
Add a note, a credit and link to the original infographic and you're set. Hit the share button.