Images often say more than words. In any case, they attract attention and are an important element to activate users in email marketing. No wonder that images are an elementary component of designed HTML mailings in email marketing. However, not all images are the same. Different file formats are available and some are more suited than others for email marketing.
JPEG, GIF, EPS, BMP, PNG, PSD etc. – the list of file formats for graphics could be continued for a long time. The image formats differ, e.g. in file size, compression and also display in common email clients. The most common formats for designed HTML mailings in email marketing are JPG, GIF and PNG.
JPEG (Joint Photographics Expert Group)
The abbreviation JPEG (file suffix mostly JPG) describes different types of image compression. Although there are lossless methods, usually compression methods are used, which result in a loss of quality compared to the original file. In the common image editing programs (e.g. Photoshop), the degree of compression can be specified and with it the file size. The more compressed an image, the greater the quality losses. These are expressed in artefacts, i.e. different types of image anomalies, such as blur or block artefacts (see screenshot). Artefacts are especially visible in images with well defined edges (e.g. texts) A great advantage of JPEG formats is the high colour depth. JPEG images can display up to 16 million colours.
Conclusion: The JPEG format is especially suited for images with many colours, e.g. photos. Users expect that images in emails load without a delay. Especially for users with slower network connections (mobile users), highly compressed file formats, such as JPEG, are a good option. However, in the case of images with defined edges, particularly images containing a lot of text, another file format might be more suitable.
GIF (Graphics Interchange Format)
GIF is a compressed format without losses, however, it can only display a maximum of 256 colours. In addition to colours, GIFs can also contain transparent layers, i.e. GIFs may be placed before other graphics without showing the GIF edges (screenshot). Furthermore, GIFs can be animated.
Conclusion: The GIF format is recommended for simple graphic displays, such as logos. Transparent GIFs would be suited for changing background graphics, e.g. a header, which changes in every edition of the newsletter. Animated GIFs are an alternative to video emails, which are not (yet) supported by many clients. As GIFs are compressed without loss, you must watch the file size.
PNG (Portable Network Graphics)
There are two types of PNG format, which can both be compressed without loss: PNG-8 and PNG-24. While PNG-8 only allows for 256 colours, PNG-24 can display several million colours. PNG also allows transparent layers, but cannot be animated. Some older browser versions (e.g. Internet Explorer prior to version 6) are not able to display PNG correctly.
Conclusion: PNG-24 is particularly suited for images with many colours, which show artefacts in JPG format. As PNGs can be compressed without loss, the image files are larger than with JPGs. This may result in a delay of the display when the email contains more than one image.