Are there any restrictions or best practices for using images in email signatures?

Question

Layout of letters

Situation: A company is developing a new corporate email signature standard. There are suggestions for the use of images.
Are there any restrictions or best practices for using images in email signatures: what to use and what not to do. The question is of interest not from the point of view of how to do it, but from the point of view of display on various devices and clients, especially mobile ones.
For example, it is undesirable to use a logo / photo, because when reading letters from the phone, the element is not loaded correctly? Or you shouldn't integrate icons that link to company profiles in social networks - again, only links are displayed on the phone, and not the icons themselves.
There may be some restrictions on the overall size or width of the caption.

Answer 1

You remember the main thing - that viewing mail from mobile devices assumes that all these beauties will be downloaded depending on the desire of the client. Your letter and signature must be legible and well-formed WITHOUT pictures. refuse links to the Internet - only attachments. Well, without fanaticism. 4-5 pictures will be enough if you do not have an entertainment gaming portal, but business correspondence.

Answer 2

A very strange desire for 2015. All sane email clients have long supported user avatars (gravatar), be it web interfaces or desktop and mobile clients. On Windows it's weak, yes. But on osx, android and ios - everything is fine with this.
And the signature can be issued without html.