It may sound odd, but my wife and I had an issue with images in SMS on WP8.1 (930 and 830). A couple of weeks ago, images stopped working all together on both phones at once. Figured it was something on ATT's end, but my wife swapped her sim back to her S6 and images were working like normal... Anyhow, wound up having to manually enter the MMS APN on both phones to get images working on SMS... IDK if ATT or MS changed something on the back end that caused this??? Or maybe it's something local with the cell towers in San Antonio, TX???
Anyhow, if you happen to be on ATT and having image SMS issues, you might give manually setting the MMS APN settings a try...
Here is the web page that those settings are from...
http://www.att.com/esupport/article.html#!/wireless/KB424489?source=redirect