I never had this problem with earlier versions of eM Client. When I try to insert a 1920 x 1080 image into an e-mail, it is instantly downsized to “Medium” I can right click on it, and select “Original” and the picture does resize, but the next time I try to insert an image, the default size has been reset to Medium again. I selected “Set Default”, and the settings menu opens, but nowhere is there a seting to set the default image size. All I want to do is set the Default to “Original”. With previous versions of eM Client, I never had to give this a thought. I imported a 1920 x 1080 image, and that is what showed up in the e-mail, not a downsized version. This only started being a problem with this latest version of eM Client.
Go to menu/tools/settings/mail/compose. At the bottom, under “Image size” the is a pull-down to set the default, including original. 
Thanks, Hadn’t noticed that I needed to scroll to get to that last option. I recently had to alter my zoom level for pages, and so that menu was hidden where it had not been in the past. The trouble is, that when I opened this menu, I already had set it to “Original”, but images added to an e-mail are still set to “Medium” and the right click menu on the image shows the default is set to “Medium”. If I click on Set Default, it brings me back here, where it says the default is Original. This was not an issue until the last update.