Can you try to email yourself a photo and look at it on multiple PCs? It could just be a problem with a particular PC. How are you attaching the photo to the email?

To be honest, I would recommend that you just take the photo and manually attach it to the email. Don't drag and drop it or copy and paste it into the body of the message, that can cause issues such as quality loss. I would create an email and attach it as if it were a txt file or mp3 that you want to send.