Herein lies the answer!
Since you can't disable the stock mesaging app that comes on the LG G3, you have to treat it as part of a 3rd party app ie: Textra.
If you're having problems sending MMS pictures with Textra on the LG G3:
First make the stock messaging app your default app then in the stock messaging app, go into SETTINGS>NOTIFICATION SETTINGS>uncheck the NOTIFICATIONS - Display new message notifications box.
Go back to Textra and make it your default messaging app when it asks you and it will now send pictures!
See screenshots:
This drove me crazy for 72 hours until I pieced these clues together. Hope it helps!
I don't think the messenger app can be disabled in any android because (from what i've heard) it's the backend for 3rd party messenger apps. If you disable it, chomp and handcent etc wouldn't work either.
If you've disabled notifications in stock messenger app then i can't think why you should still be getting them. I'm using chomp too and i have my notifications switched off in stock, so i'm only getting notifications from chomp.
I guess it must be network related
Since you can't disable the stock mesaging app that comes on the LG G3, you have to treat it as part of a 3rd party app ie: Textra.
If you're having problems sending MMS pictures with Textra on the LG G3:
First make the stock messaging app your default app then in the stock messaging app, go into SETTINGS>NOTIFICATION SETTINGS>uncheck the NOTIFICATIONS - Display new message notifications box.
Go back to Textra and make it your default messaging app when it asks you and it will now send pictures!
See screenshots:
This drove me crazy for 72 hours until I pieced these clues together. Hope it helps!