Based on the results of several tests we conducted, the inclusion of additional images does not increase response rates to advocacy messages or fundraising appeals, nor does it decrease them. While it may not hurt to include properly formatted images in your email message (thought it could hurt if your message gets quarantined or rejected by a potential donor or activist because it looks blank in the preview pane), it also does not appear to help at all.
However, if you've got a great photo of a charismatic or cuddly animal, beautiful landscape, or appealing person making direct eye contact, you may want to use it - provided you take the time to format your image properly:
1) Use smaller header images. Large header images may take a long time to load. They also push the text of your email message farther down the screen, forcing readers to scroll down to read your message or click on the all-important links to the action or donation page. To minimize scrolling and loading time, make sure that the first few paragraphs of your email message (and at least one link to the action or fundraising page) are visible above the fold in your HTML message.
2) Always include the image dimensions and alt text. With so many email providers using image-blocking technology these days, HTML messages that don't include the image dimensions or alternative text can leave your messages looking completely mangled. Specifying image dimensions will ensure that the appropriate amount of blank space is left in place of the image. The alt text (text that appears in place of a blocked image) will clue your readers in to what they should be seeing. However, if you use spacer images (and we don't recommend them), do not include alt text!
3) Cut back on spacer images. Image blocking makes it less attractive to use spacer images as they will manifest themselves as unnecessary little Xs in empty boxes.
4) Avoid image overload. While it can be tempting to jam pack HTML messages with images and photos, the more images you include in your message the longer it will take to download. For your list members with dial-up or other slow Internet connections, this can lead to a frustratingly long wait time to read your email messages! Try to avoid sending image-laden messages and be sure to keep the file size of any images you do include to a minimum.
5) Consider adding an unobtrusive "view web version" link. This link offers people an option to view the message as it was intended to look. However, keep in mind that this link does constitute another (unnecessary) barrier between your user and whatever it is you want them to do. Ideally, your message should be created so that it's not necessary to include a link to a web version.