Text as Images
Using text that is important to users within images is generally considered bad practice as it adversely affects findability, usability and accessibility.
Findability
Search Engine Optimisation
Search engines scan through text-based content within a website to compare the relevance of the site to what a user searches for. If you put your keywords into images, search engines will be unable to find them. Even though you may add them as "alternative text", search engines will always find websites by their page headings more easily than words in their images.
In-page Search
User search within your page will find keywords they are looking for in their text rather than in their images.
Usability
Users may wish to copy and paste text from your website or save it to an app that does not support images. Rendering text as images may make copying the text impossible.
Accessibility
Visually-impaired users may use "Text Sizing" to enlarge or shrink text on a page which is difficult to read. However, this will not work with images, and they will not change size.
Fonts and styles such as difficult-to-read colour contrast combinations may be difficult for some users to read. If text is in HTML, the user can disable these styles in their browser. However, any text rendered as images will never change style, making any hard-to-read text impossible to refine using the browser.