Welcome to the forum!
Do you understand the difference between raster graphics and vector graphics?
Are you going to send the decal paper to some kind of digital cutting machine, to cut out the decal? If so, then that's probably why you've chosen Inkscape (or it was recommended). If not, then you would be better off using a raster graphics editor, such as GIMP or Photoshop.
Inkscape is a vector graphics editor. It can create the paths (type of lines) which are needed by cutting machines. But if you don't need vector paths, everything you need to do, can be done better by a raster graphics editor.
If you do need Inkscape for making the vector paths, then some part of your work could still be done in a raster editor. But how much, I can't be sure without seeing the photo, along with an explanation of what you want to do. For example, which parts do you need the cutting path for, and which parts need the colors changed.
It might be possible to use a trace engine, to create the cutting paths. Or you might need to manually draw them. But I can't suggest which way would be better, without seeing the photo.
If you can't share the photo, for some reason, maybe you can find a similar photo or image?
