This warning means that you do not have on that page an image that is suitable for use in AMP structured data.Google requirements for that image can be found in their documentation on this page
In short an image can be used there if:
- it's 1200px wide or more
- and it has more than 800,000 pixels
- and it's found on the page itself (ie you cannot just put any image you like in your structured data)
When your page has an image that meet those criteria, wbAMP will insert it in the structured data.
As a side note, invalid structured data is not a problem for AMP which will be valid and Google will use your pages normally. The thing you are missing out on if your SD are not valid is the AMP structured data carousel. This might or might not be important to you depending on your particular site. The carousel is only displayed for certain types of results, for instance news or products, when there are multiple responses available from different sites and probably other things.