Learning graphic design may be one of the most useful skills for an online marketer assuming you are starting off with a minimum of artistic skills. Many people with no real experience in art get into graphic design and try for quite some time before figuring out they simply lack the eye to actually develop the sort of images that grab attention and get results. It can be a frustrating experience.
I wish it was the case that every person getting into graphic design would have a positive experience, but it can be very frustrating to try to develop these kinds of skills when you don't have an aptitude for art. As a professional copywriter and someone with a modicum of graphic arts experience both for personal projects and in an outsourcing environment - all I would say is don't bother unless you love to MAKE things.
This is the primary critical skill needed to be good at graphic design. So be sure to ask yourself that question, "Do I like to make things?"
From a purely business standpoint - particularly in online circles - being able to create graphic design content such as banners, themes, templates, graphics packs, etc... is one of the single easiest ways to earn a living online. I've made most of my money online in CPA, but a huge chunk of earnings has come from outsourcing projects such as writing, video, sales thread design, etc... So I can definitely say that it's pretty easy to get paid if/when you develop the right graphic design skills.
Finally, all the others are right - you can get the best training in graphic design online for free in hundreds of places including youtube, udemy, dailymotion, and tons of other sites so there is little sense in spending the money for a course. This is particularly true for someone with limited graphic design experience. You should try some of the free tutorials and develop some base level skills to see if you even enjoy it. If you want to spend the money after that - go for it, but most likely you will discover you can easily acquire these skills for free.
Good luck!
