I often send screenshots to friends using OS X’s Messages app by pasting them into the section of the window where you compose text; they can be used to illustrate things, such as problems on my Mac, or show bits of text or images from the web. It’s a practical way of exchanging information.
Since the release of OS X 10.10 Yosemite, however, this function has not worked reliably when sending screenshots via AIM accounts (it seems to work with iMessages). When you sent a screenshot before Yosemite, you’d see the progress of the screenshot in Messages’ File Transfers window. But in Yosemite, the File Transfers window shows the screenshot as Sent, even before it is fully transferred. So if the screenshot doesn’t get sent, you won’t know it until you ask the person you sent it to. (This only applies to screenshots that you paste; screenshots that you have saved as files, and that you drag to the Messages window, send reliably.)
And screenshots fail often. With one friend, we have to ask each other every time we send screenshots to make sure they’re sent, because there’s no feedback. When this happens, you have to force quit a background process to get it to work again. Quitting Messages alone won’t work.
To do this, open Activity Monitor, which is in your /Applications/Utilities folder. Click in the search field, and type “imagent.” Click on that process to select it, then click the x button at the top-left of the window, and then click Force Quit.
This quits the imagent background process, and also quits Messages. When you relaunch Messages, screenshots should work again.
Rob Griffiths has explains how to write an AppleScript that can simplify this procedure. Just run the script and it will kill the imagent process.