Summary
Avoid using images of text. It injures kittens.
Slightly longer read
Actual text is almost always better with respect to accessibility. If you are using graphical text embedded within a larger image then there are restrictions:
- As the image becomes smaller (think mobile phone) the text will, most likely, become unreadable. This is especially true of users with low vision.
- For people using a screen reader the text is only readable via alt text.
There are an increasing number of ways to overlay/add text to images. The Slides and Features afforded on the Libraries sites are created, in part, to accomplish just that.
Contact Ken for more information about adapting your message to a more accessible format.
TL;DR
More more information refer to the Web Accessibility Initiative.