for information we have done all design with paint.net It’s not very good but it’s very easy to use
I would also look at http://www.gameart2d.com/
They offer some freebies and some of those are UI elements that you can use for a lot of buttons. For making your own:
Adobe Photoshop – as low as $10/month with their Photographer Bundle or part of the $50/mo full Creative Cloud suite. or $20/month single app. (raster)
Adobe Illustrator – $20/month single app, $50/month full suite. (vector)
TheGimp – free (raster)
InkScape – free (vector)
Paintstorm Studio - $25 one time, raster
Autodesk Sketchbook Pro $5 / month, $30 / year (raster) <---- really good drawing tool.
Adobe Fireworks, which allowed for both raster and vector images in the same environment, has been discontinued by Adobe. However, I’ve found a great replacement.
Sketch (free trial / $99)
It has a great workflow and I used it quickly generate all the sample art for my Dynamic Shader demo