It's possible that your images are in NEF file type instead of jpg. My own mail client won't send a nef (RAW) file type and says it's not the right file type. Could that be what you're experiencing? When you look at the list of photographs, what is the file extension of the photographs (the letters after the dot)?
If they are jpg files, they may be too large, as was previously suggested. You will need to resize them before emailing them. If they are nef files, you will need to convert them before emailing them, I believe.