Why is it better not to send an image-only newsletter?
Many email clients do not display images immediately: your subscribers have to load them manually first. If your newsletter consists only of images, there is nothing to read at that moment and subscribers are more likely to lose interest. You can read where images do appear immediately under why images are not always immediately visible.
Plain text solves this. It allows you to persuade your subscribers to load the images after all or to open the webversion. This increases the chance that your message comes across even without images and that subscribers click through to your website.
Therefore, use images to complement your text. When designing your newsletter, check whether it is still interesting without loaded images. Adding alt text for each image helps with this: that text is shown in place of the image for as long as it has not loaded yet.
Text is also important for spam filters. They look at the ratio of text to images. If your newsletter contains relatively little text, it is more likely to be marked as spam.