Using text rather than images has multiple advantages:
Downloads faster - a users patience extends to about 7 seconds.
Cheaper to update - a simple find and replace.
Discoverable by search engines - if people can't get to your site, what's the point of it being there?
Readable on an iPhone - Images will either be resized or require scrolling, which would create difficulties for readability.
<img src="Images/Heading_Welcome.gif>
❌ Bad Example - Using image for text header
<h1>Welcome to My Page</h1>
✅ Good Example - This text can be easily rendered by the browser and recognized by search engine