Here are some basic rules to properly constructing an eblast with regards to images and text:-

• Every image should have brief alt text explaining to non-image users what belongs there.
• Do not use images in place of header text. Users with images disabled need to be able to read those headers. In this case alt-text is not a sufficient replacement.
• A header image is good, but it should be relatively narrow (in terms of height). Users should be able to read your first paragraph, your lead-in text, without having to scroll down.
• The entire eblast should be readable and compelling without images — avoid giant gaps in text where large images should be.