This, and don't use transparent images without setting background either please! I've gotten emails from large companies that were completely white, because someone assumed a certain background color

Transparent images can also be a problem outside emails, image viewers can have different background colors.

I had this issue when I was working on a embed project (similar to embed.ly), some websites had transparent image (in open graph or schema.org) and not all of them were expecting white background, so I have to crawl the page and try to detect the background color best suited for the image. This didn't work well (and required crawling the page) so I ended up doing edge detection on the image (easy with transparency) and average the resulting color to find the best background.