We had preexisting issues with emails rendering correctly in various clients (Outlook 2007... cough cough) and it was taking hours to get the bugs fixed (if at all) -- which severely delayed our campaign launches. I researched resources to help our emails render correctly and discovered Litmus. I presented the service to my boss, as well as showing documentation of all the benefits we would get out of quickly making sure our emails were rendering correctly.
<span> with the inline CSS works in Gmail for text, but does not work with images.
In order to hide an image in desktop (that you want displaying in mobile), you need to wrap the image in a <div> with the styling.
I tried wrapping my image in a <span> tag, which failed to hide in Gmail (desktop versions) and Google Apps.
Wrapping the image in a <div> with the inline styles works.