In terms of resolution we recommend of minimum of 96dpi, and 120dpi for best render.
This recommendation also is valuable for all logos and other images in Templafy.
We don’t particularly have recommendations in terms of dimensions. However, a lot more people read their emails on smartphones these days so you want to give them a nice experience as well.
In this case, it’s best to look at recommended image width for smartphones. For example, one of the smallest screen still on the market is the Iphone 5 and that requires a width of 320 max (for best fit of banner on screen).
Finally, for format recommendation, it’s always best to use png or jpeg but gif works fine too. Keep in mind that we don’t support transparency so if you do come across a situation where you need a background color for your email signature, you will have to set the same background color in the banner (to avoid having a white square).
HTML best practices
Follow our HTML best practice guide when setting up a banner: https://support.templafy.com/hc/en-us/articles/360000227365-Email-signature-HTML-Best-practice
You can add UTM codes in the banners to allow tracking of click rate, session time and bounce rate.