Spiders can’t read images, so you should replace images by text whenever it is possible. It is understood that sometimes design requires graphic headers and images, but if the image only contain text, you should consider using text instead.

Few things to consider when creating your images:
- All your images should have alt attribute
<img src="some-image-file.jpg" alt="" height="50"" width="50" />
- All visible meaningful images should have alt attribute text
<img src="some-image-file.jpg" alt="some great alt text here" height="50"" width="50" />
- Alt text should be representive to what the image is about
- Use keywords in your alt text but avoid stuffing
- Try not to exceed 5-7 words in your alt text