GIMP is image editing software, with what seems to be quite a massive following. There are lots of resources out there to help you with it. AND IT’S FREE!
Here is a link to GIMP for Macs. I like that this is available without windows. You can also get GIMP for pcs and macs with windows.
These GIMP beginner tutorials are great to begin, but I found this GIMP tutorial for designing and coding a site REALLY good! I learnt how to create a site and use GIMP in one go.
Help Developer has many other good GIMP tutorials too so check them out!
A superb resource is GIMP talk – a forum for GIMP users, which will answer your questions but also has some good tutorials.
If you decide to try GIMP, I’d love to know how you’re getting on!
