I am not sure if you have tried any of these methods before.
1) Restart everything. Turn your phone and your Wi-Fi router off and back on to clear out temporary glitches.
2) Try to see if you can post pics using incognito browser.
3) Clear the cache associated with Chat Avenue. Can see the pinned thread on Technical Support on how to do that.
4) Turn off any active VPN or security proxy to see if your Wi-Fi lets the picture through.
There is no guarantee this will work. Your Wi-Fi network may be blocking media uploads, your router's firewall might restrict app data, or your phone's Wi-Fi calling/network settings are misconfigured.
Best of luck!