We are having an interesting time (since yesterday morning) with email signature images in Gmail at the moment.
It's a minor thing, but it reflects poorly on an organisation's digital corporate image (and gets marketing people really hot under the collar).
Google seems to have changed something, somewhere about how signature images in Gmail work, at least for some users/organisations.
Apparently, this affects "a small number" of their clients, according to their tech support (presumably, they change a thing, roll it out to some of their clients and servers and see what happens... and we're in the guinea pig bunch this time).
What you see, if this affects you, is "broken image" icons in emails received by clients and in your own sent items (and existing email conversations) instead of your signature image.
Neither of the work-arounds their tech support suggested... worked.
But I seem to have figured out what *does* work...
This morning, I then thought "hmm, what if it's as stupidly simple as a permissions problem?".
Sometimes, your brain needs a bit of ...time to figure out a problem; I've lost track of the number of times I've fixed an intractable problem the next day after a good nights sleep, or a thing randomly hits me in the shower - a time of day my mind it typically fairly calm. Sometimes, it's because you read a thing, which was part of what triggered this "brainwave" (a post saying you had to have "public" permissions on an image for it to "work" in Gmail signatures).
Change the sharing permissions to "public on the web" to your signature image in Drive and re-insert the image from Drive into your signature block. I suggest renaming it to include "pub" or "public" so you remember it has permissive permissions. A bit of a pain, but if signature image "branding" is "mission critical" at your site, then this seems to fix the issue.
Quite why the "default" permissions on uploading a signature image through the signature image box itself are suddenly broken is a bit... odd. (I tried that and it did exactly the same broken thing), but at least we have a working work-around now, even if it requires some manual intervention. One of the joys of Google's de facto "live life in Beta" mentality, I guess!
The other thing that seems to work is just hosting the image on a web server somewhere, which isn't exactly a neat solution if every single person has their own customised signature - but would work quite well for generic corporate branding, and then carries the bonus that you can just change the imagery any time that you need to by replacing the relevant file(s).
I've suggested the root of their problems might lie in a permissions snafu to their tech support. We'll see if that doesn't suddenly get a fix worked up...! It seems like the sort of thing that should be "obvious" to engineers of their calibre, but we all have those days...