There is no problem with the signature or the GIF (and I'm unsure why you'd even think that). I see the problem being with the "improvement" in the update. Before, I could very easily put the image (GIF/JPEG/whatever) into the PDF document. I have attached an example of the finished result.
Now, for some reason, the updated program doesn't do it the way it used to and I get a screen that appears (see original post) that I cannot get past. If the "improvement" is now that I have to sign in to Adobe to get past this, it's pretty annoying and far from an improvement.

Thanks for sharing the further information.
Adobe Acrobat official update (19.008.20080) containing the fix for this issue is now available. This update will be automatically pushed to all existing installations of Acrobat. If you want, you may also manually trigger the update early by opening the application and going to Help > Check for Updates.
19.008.20080 Optional update, October 22, 2018 — Release Notes for Acrobat DC Products
Let us know if this fixes the issue.
-Akanchha