How are you storing your photos on your Mac? By manually copying them from a memory card into a file structure of your choosing or by importing them into the Apple Photos app?
If you are doing it manually, then when attaching it to an email, don't select the PHOTO option, select the attachment option (usually looks like a paperclip). That way Finder will open and you can navigate to the folder you want.
If you are importing through Apple Photos, then the file structure within the Photos library is not intended for users to directly access. You are better off finding the photo you want in the Photos app, click the SHARE button at the top right (looks like a square with an arrow pointing up) and select MAIL or MESSAGE. Or else you can export the photo(s) to the folder of your choice (I have a folder set up called Photos Exports) and then attach them from there.