The skins and texture packs that I make are destined for the Minecraft marketplace. I don't personally have an account that has been approved to sell on the marketplace but I work for one of the larger content creators on there. I landed this gig through one of the freelancer marketplaces over a year ago, but after a few months of working together, we decided to take our working relationship off that site and communicate over skype/pay via paypal. This worked well for me as I have a good profile online, and I got lucky with that job, but it probably doesn't apply to most freelancers, as there isn't a constant stream of Minecraft gigs to be had. An easier way to score some texture work would be as follows:
If I we're to be looking for some Minecraft texture work now, I would take the following steps: Firstly, I'd get together my best skins and textures and present them nicely by taking some screen shots in-game. I'd compile these in one place on a web page so they are easy to view. Then I would look at the Minecraft marketplace and research some contact info for the creators on there. I would then reach out to them with a link to my Minecraft portfolio.
Texture packs are time consuming. It takes me over a month to make a full pack, so there is demand for people who can make them, and there is plenty of work available. If you know what you are doing, can work fairly swiftly and make some good quality textures, it's not difficult to get the work.
Skins or Texture packs
I started off by making skins. This was a easy job, and most designers would find it easy, therefore, there is a lot of competition, and the packs in the marketplace have to really stand out.
Now, I exclusively work on texture packs, this is basically a reskin of all of the blocks, items, mobs etc. It is more time-consuming and difficult, but there is less competition for me as an artist, and for my client also.
Tools
For designing Minecraft texture packs, you don't need any special tools. I predominantly use photoshop, but you could use GIMP. This is fine for all of the blocks and items. for the mobs, I use novaskin which is a website with an editing tool for all of the mobs and entities. I also use blockbench for any mobs that aren't on novaskin. blockbench is free, as is novaskin.
You can download the entire resource pack for vanilla minecraft on the minecraft website, this works as a starting point for your designs, and also contains the models for use in blockbench.
Does it pay well?
I'm happy with the amount I receive for this work. It is really reliable and pretty stress-free. It isn't going to make me a millionaire, but I like it.