To add imagery to a signature, you have two options available to you. You can either add a logo/image by embedding it directly into the signature or hosting it externally and linking to it.
The vast majority of email clients display logos/images in an email signature without any major issues. However, there are benefits to choosing one method over the other.
If you want a logo/imagery to appear without having to click a 'Download Images' button, we recommend embedding it all within the signature. When an email client refers to these images, they will all be displayed automatically. This is because they are referenced in the source of the message.
However, embedded images will slightly increase the size of your emails and can appear as attachments within mobile clients, particularly in iOS (iPhone and iPad).
If you don't want imagery to appear as attachments in mobile emails, as mentioned previously, we recommend saving and hosting your logo/images on a web server. They will then be referenced in the email using a web URL.
By doing this, your images are never actually a physical part of the email. Instead, they are downloaded into the message every time it is viewed. This has the added benefit of making the email file size smaller.
In some cases, a recipient may need to manually click a 'Download Images' button for this additional content to be displayed. This is a way of protecting the recipient against possible viruses.
Which option should I choose?
You need to consider how your customers/clients view the emails you send them. If a large percentage use email clients like Outlook, then embedded images are probably the way to go. If, however, you know that a lot of recipients will view your emails on mobile devices, we recommend you use hosted imagery.