Screenshots not sending from Mac via Messages

I am having a new issue sending screenshots, captured as png images, via Messages from my Mac. The messages are marked "Not Delivered." Through testing, I have determined that the issue is not dependent on the recipient, the type of screenshot (i.e. a portion of the screen, a specific window, or the entire screen), or the name of the screenshot. If I Airdrop the image to my iPhone, I can send it normally from my iPhone, but if I then download that same file from Messages to my Mac, it still will not send from my Mac. The file will send from my Mac if I first import the image to Photos. If I convert the file to a jpg or other file type, it also will send.