Please see this Stack Overflow post for full details on this particularly tricky bug. Let me know if you have a trick.

http://stackoverflow.com/questions/43770025/android-gmail-app-horizontal-1px-gaps-between-inline-images-images-in-td

In the Android Gmail app, there is sometimes 1px of space between inline images (<tr> with multiple <td> that contain images). This will push the rightmost image outside the frame of the email.

screenshot