One of the greatest things about email is that you can present your information to a targeted audience and you can spend time on making the emails attractive enough to capture attention. One of the elements in these email is graphics, and these elements can be an awesome addition or they can be the death of your email. As in all things, there is a balance, and we will discuss some of them.
Number of Images - When embedding images, each image in the email becomes an attachment to the email. Because of this, each email will have to be read from the disk, encoded, then appended onto the email message and this adds to the email's overall size too. If you have 10 images, then it has to do this 10 times. I have seen some emails with 100 embedded images in them. Creating the email with lots of embedded images is one thing, but the people receiving the emails will have to able to read them too and they might have limitations themselves. I tell people if you start exceeding 20 embedded images in emails then you are pushing it. Yes, the technology at both ends will handle it, but it is a matter of practicality.
Size of Images - We have had some clients call and say their emails are going out so slow. After analyzing it we will find they included a single embedded image that was 4 megabytes. Now it is possible to send and receive this size of email, but as the email size goes up, the amount of time its takes to send increases too. Other things to consider is the recipient's email system may reject these email sizes, or the recipient may still be on a dial-up or slow DSL and will not appreciate your email taking 30 minutes to load. This is a huge consideration especially for international email lists. So, consider this, you may have an image that is 4 megabytes because it is super high resolution. For an email, you are not going to need that resolution or size, so open the image in an image editor and reduce its size so it fits in the email space nicely. You will find these images will reduce in the the 10-90KB range and now that is a good size to send.
All Email Clients Not Created Equal - There are several email clients that will just not show embedded images. May portable devices are like that. When you create your email, test it will several different email clients like gmail, yahoo, and hotmail. Also, check portable devices like Androids, iPhones, and iPad to make sure the email looks like you intended.
Having to post images to website - An alternative to embedding images it to post the images to a website somewhere and reference these images in the email. The advantages to this is that your email is very small and will deliver quickly. Images from webservers load very quickly, much faster than an email system. The disadvantages of this is that your email is not a complete unit on it's own and the recipient will have to be on the internet to see the entire content of the message. This is usually not a problem though because you are usually on the internet when reading email anyway. Also, you will have to coordinate the availability of the images being on the website for the email's use.