Saving image from gmail app
In your Gmail app go to settings, then click on your your target email address (where you want to get image attachments downloaded) , then untick the option which says "Auto download attachment to recent messages via Wi-Fi" try again to get your image.

It seems attachment downloads are restricted to wi-fi (default) but option is not clear at all as it seems to imlply a different thing. So disabling this option implies both data and wi-fi are considered (although the option is a bit misleading) It however fixed the issue.

Edit

Downloaded images are saved in downloads folder of sdcard or here /storage/emulated/0/Download

credits: beeshyams