There were some issues with twitter images in earlier versions, but 1.7.8 should be fine (still upgrade to 1.7.9 would be recommended)
I assume the images are not visible in both designer and player?
If so could you enable debugging in CMS and then check CMS logs page please?
Before you upload the troubleshooting.txt here, please make sure it does not contain any sensitive data.
You can also double check curl - Using Tickers, Forecast, Twitter and other external resources that make use of HTTPS connections
Although since feed text is working I’d imagine that is probably fine.
You can also double check filesystem permissions on your web server and make sure CMS can access everything it needs.