I found GIMP impossible to use. There is another similar program to GIMP that I tried as well and found it impossible. But I was already accustomed to using Photoshop and there are things that GIMP and that other thing just can't do.
I'd suggest using Adobe Cloud. It's the best out there and what 99.9% of professionals use in photography, design, illustration, ect. It's 50$/month and you can start/stop anytime. You have access to all of Adobe's products and Adobe updates them all regularly.
50$ may seem like a lot to spend per month on a hobby but mastering Abode is a skill set that you can absolutely rely on in your career, assuming you love it enough to want to go into design. A certification in mastering Adobe products is a huge asset and stands out on any resume, and you if you've used it as a hobby for your growing up life, that certificate will be a breeze to get once the time comes. 50$ spent to learn that you don't want to get really involved in design down the road is also money well spent I believe. Some people spend their whole lives wondering what they want to do and figuring out what you don't want to do is a huge step along the way.
Edit: as for Flash, don't bother. It's becomign obselete and won't be around too much longer. Most developers use HTML5 and other coding languages now instead because most of the newer softwares out there don't support flash or only partially support flash. Trust me, my boyfriend is a developer (has been for 20 years) and he hates when his customers insist on using Flash and then complaining why it won't work on all of their devices.
If that is something you are interested in, I'd recommend checking out some coding. Start with basic html & css (it's really basic and easy to learn and see your code working) and branch out into html5. The easiest way to start learning this for free is to get a blog and create your own layouts.