I would suggest using Gyazo. Open up the screenshot, If you want it cropped (Or re sized) Hold down left click and size it the way you want. If you don't want it cropped, Just hold left click and make sure the whole image is grey. Wait a min., A new tab will pop up with your image in thed browser that you downloaded gyazo on. Then, Right click the image and select "Copy Image" Then go into your sig., Right click and click "Paste" and your image will pop up, When you're done, Click save changes. Hope this helped!