Sadly, neither language is appropriate for a desktop image editing program.
You haven't told us which desktop you have in mind, I'll assume it's either Windows or Mac.
Ruby is not appropriate because it fails 2 of your requirements:
- it has a terrible startup time because at startup it has to initialize a rather complicated VM, which involves loading quite a big part of its standard library
- it's very slow (compared to C/Java/Go) doing the kind of computations that image processing entails
Go is statically linked and is compiled to machine code, so its startup time is excellent and the speed is close to C (i.e. it's the fastest language you can hope to choose after C/C++).
However, Go has no support whatsoever for writing Mac desktop apps (i.e. it has no bridge to Objective-C/Cocoa runtime) and the support for writing Windows desktop apps is extremely poor.
If you're doing Windows, the only language that gives you fast startup time is C/C++/Delphi. C# might have acceptable startup time and it's fast enough for the task (very popular paint.net is written in C# and you can find an old version of the code which is BSD-licensed and re-use a lot of its code).
For Mac, I would recommend Objective C - it's the native language of the platform, best documented and with the best, free dev tools (XCode). You can use https://github.com/philippec/Pixen as a starting point.