From personal experience (might not apply to other people, though) I can tell you pixel art is one of the most difficult types of digital art. Perfecting a design is, in most cases, removing every unnecessary element possible, until you have the simplest form of expression for your idea. And expressing something in a few pixels is an herculean task!
Unless you are interested in adding digital design / art to you pool of skills, I'd consider maybe looking for a partner that can do the graphic part, or buying some nice stock vectors and working around those. Illustration is fascinating, but like everything, it takes time. Adding to the 'perfecting' of what you will create, you still need to learn the tools to do it, so the curve can be quite steep at first. And if someone has done it already, and successfully, you could be investing that extra time you'd spend in graphics, in developing your game more.
Having said that, if you are familiar with the drawing tools Flash comes with, it might be a bit easier. I'd recommend you start by choosing a style you like (from other games for example) and try to imitate - without stealing, of course ;) - what they look like. Once you got the basics you can work on finding your own style.
