I have the same issue. It took me awhile to adjust but I have learned to hit the delete button on much of it. I have some friends that send me emails that do NOT display on my computer. The images or jokes just do NOT come up at all. I have tried to tell them this or even tried to change my setup. It just hasn't worked. I get very frustrated at it.
My excuse to my friends that ask me about an email is "my computer couldn't download it properly and it wouldn't open". If I have seen the email before, I will tell "I've seen it before and it is funny!" I don't need to tell them that I had deleted it. If they try to send it to me again, I may reply to their email saying "thank you" but not opening the images. You can always say you "forgot about that picture".
Another thing you can do is setup another free email account somewhere. Send those emails to that account to save them. That way your normal email account doesn't get overloaded and you can recall that email at a later point when you have time to open it. You can then delete the original copy. I have about 3 free emails sites I use just for identity protection purposes alone.
By the way... ALWAYS MAKE SURE THEY DON'T HAVE THE FEATURE ACTIVATED TO TELL THEM IF AND WHEN YOU OPENED THEIR EMAIL! There are some "receipt of email" some people attach to their emails to see if the receiver opened the email. This is handy for work purposes but NOT personal. If they are someone who monitors their mail like this, then open the email with one click and then delete it from there. This will let them get a receipt you opened the email BUT NOT let them know you deleted it.