And if so, I also read that the signature is not "dynamic" in that if we delete a signature, old emails sent with it won't break. So it's like a static image (with HTML?) added to the signature in our email program (I'm a Zoho user so I guess I'd have to add it manually).

That said, does the image show up or not if the end users email client is blocking images? Like do they have to click the button to "show images" to get these signatures to show up?
If their client is blocking images, does anything else show up in its place where the image should be? In other words, if the client blocks the image, do they see no signature at all?

What I'm ultimately asking here is if the signature gracefully "falls back" when images aren't shown, and falls back again if the end user uses plain text instead of HTML.

In a similar vein, do you have any sort of document list of problematic email clients? For example do you know which email clients don't show what sort of signature features well? And how to deal with it? For example I'd rather make a signature that avoids the most problematic features regarding images, styles, alignments, etc. This way I can build the most widely acceptable signature possible.
I really don't need super fancy features because in longer email chains I don't want my massive signature being sent over and over again. I just want a nice looking clean and minimal signature that works on as many clients as possible.