How to change Gmail settings to automatically unhide pictures in messages
I am not sure what version of Gmail you have, but the version I have (along with Android 4.1.2) allows me to "Show Pictures" in the message, then I have the option to "Always show pictures from this sender".



The option in the settings is to "Remove" the always show that you have set for any sender in the past.

The only way to always show images is by first showing them for a sender, then select the always show. And AFAIK, this can only be done per sender.