Many people don’t know that their email campaigns are immediately sent to spam (also called junk) because of their images. It is common best practice to ensure that there is an adequate amount of text in proportion to image content in an email message. Mailchimp recommend a maximum 20% image content to text content in a campaign.
We recommend a ratio of 80 percent text to 20 percent images in a campaign. Mailchimp
Whilst the ratio of text to images in a campaign is important in minimizing the chance of a spam filter identifying your message as spam, the content of the image itself is also very important in avoiding being identified as spam.
What is image spam
Most email marketing services allow sending of PNG, GIF and JPG/JPEG image filetypes. Of course, it’s simple enough, using an image editor, to add text to an image. Adding text to an image and inserting the image into an email campaign however could be reducing your deliverability considerably.
There is the technique of hiding text in an image for encryption purposes however in this article I refer to visible text being included in an email message as an image as opposed to HTML text. Below is an example of text on an image and text inserted as HTML (normal) text:
Example of text on an image
Example of text
This is text
Text on images is frequently used in an attempt to bypass spam filters that search for certain terms that are often used in spam email messages; instead of adding a false claim as text in an email message such as “guaranteed weight loss of 15 kg in 4 weeks”, the spammer will include the text on an image in an attempt to bypass spam filters that search email message text. This is called image spam.
It is a common technique used by spammers to put the text message into images. Università di Cagliari
How to avoid your images being marked as spam
There is no generally published maximum ratio of text on an image acceptable to spam filters. A good guideline seems to be keeping the amount of text on an image below 20% of the area of the image. Facebook have a very good free tool where you may test your images as to whether they may have too much text. The tool is made for testing images for Facebook advert purposes but it is great for testing images that you intend to use in your email campaigns.