I know nothing about Lightscribe but this suggests that you simply need to save your artwork (which you've designed in any suitable application) in .bmp, .tif or .jpg format. Then select the file as a background within the Lightscribe software (which, I assume, automatically converts it to .ls format):
https://support.hp.com/gb-en/document/c00208298
[See 'Use your own text, photos, or designs']
While Gimp is every bit as good as Photoshop (and free), just like Photoshop it's got an incredibly steep learning curve and probably far too complicated for what you're seeking to do. I suggest giving the free version of Disketch a go, as it's specifically designed for creating disc labels:
http://www.nchsoftware.com/cdlabeler/index.html
(Do NOT click on the big red 'Download Now' box. Instead, click on the link underneath 'Get it Free').
Once you've created your masterpiece in Disketch, DON'T use the 'Save' option, as that creates a file in the .deproj format. Instead, use the 'Export' option to select the .jpg format. Then import that file as a background into Lightscribe.