Does the image look better than that on the decal sheet when viewed with a loupe? The image looks decently sharp, albeit with modest details, which makes me question the quality of the decal sheet itself.
Failing that, I can only suggest checking and make sure any "enhancement" functions (image smoothing, etc.) of the scanner software are disabled. This way you should have more raw data avaliable to work with in your paint program. Also, on my last scanner, I had better results when using the maximum optical resolution, instead of the max software interpolated resolution.
I've not worked regularly with scanners in some years now, sorry if my advice is out of date.