My process is that I'm usually copying the images from other apps and pasting them into the email. Depending on the place it is copied from the files can be huge or tiny (and also depends if I copy it from the built in screen or my external ips display - which I find very odd). The support article (Mail (El Capitan): Add attachments) doesn't explain things terribly well.
You can't double click a pasted image and go straight to preview to resize them, because preview complains that it can't save back to the location and prompts you to save a new file. So that's paste to mail, double click, resize, close, save as, tab back to mail, select image in body, delete, drag new image into position ... way too complicated especially since I'm now doing this dozens of times a day.
Is there then a way to paste the image that is in my clipboard to a file on the desktop so that I can then open it in preview and then use the awful "adjust size" function within Preview to change the image size back so that I can then attach it to mail? Easier? Is there an automator way to do this - like can I bind a keyboard shortcut like shift-control-command-v to bring up preview, make a file form clipboard, then bring up the resize tool? I don't know how to do that.