Alt and Title Tags
Moore’s Law states that the speed of processors will double every two years or so. It seems odd therefore that when designing in direct email marketing you have to consider systems which cannot, for various reasons, display images when reading emails. It can give the feeling of a retrograde step.
If you use professional email marketing software you will generally be given the ability to attach alt and/or title tags to your images and it is easy to become confused as to which does what. Tags are is essence instructions in hyper-text mark-up language (html) to a browser or email service provider. These are defined within the signs <>.
In the case of images it will give a location where the image can be found, hence the term image tag. But it can do other things as well. For those engaged in direct email marketing, it should.
There is an ever expanding choice of ways to access emails. Most of the more recent types will not display images in emails. Any presented might be shown as a blank space in the original image size or else a small X or similar.
If your email depends on understanding of the picture then all is not lost.
Alt and title tags provide a way of overcoming the problem. Firstly to explain the difference between the alt and title tags for email purposes: if either one is used it will be shown. If both are used the title tag will be displayed.
Instead of the blank or X the tag text will be shown. The only question for you is what you want to be displayed.
Captions for images are not that good an idea. Images should be self explanatory in emails as readers scan. Do not expect them to read captions. Your image will have some relevance so this is what your tag should show.
If you were advertising travel insurance with a picture of theft of a wallet, then might seem to be best. Do not describe the image, concentrate on its function.