ledbetter

I would approach your dilemma by using a combination of lemon juice and a quality leather conditioner, like Tantex. (www.amazon.com/gp/product/B007XG6MYM). This will remove the glossy appearance you hate, but keep the product looking clean and new. The lemon juice will help serve as a mild bleaching agent, hopefully removing the sheen surface. If this doesn't work, you might need a stronger bleach like oxalic acid which is used in woodworking. After it has completely dried out, you can apply the conditioner to keep the leather soft, yet protected from the elements. Personally, I prefer the finish of a good conditioner. I think high gloss on leather takes away from its natural beauty, but to each their own. If the conditioner is applied on a regular basis it can keep your leather looking great for many years.

I've always been partial to Photoshop, but I do graphic and web design as my day job. If you plan on doing a lot of retouching and custom editing of your work PS is the way to go. If you you just need simple retouching and you're on a tight budget, GIMP should do just fine.

I also have a failed attempted at making a knife sheath. It's one of those projects that seems simple enough, but actually takes a high attention to detail. I will probably revisit the project at some point in the future.