I would recommend Photoshop - though another powerful, and free, tool is GIMP. I believe that these are pretty much the gold standard for photo editing. There may be others, but the ubiquitous use of these means there are lots of people to ask and lots of tutorials to read.
Here's your image from your previous post, edits and explanation below:
The edit from your screenshot was actually very straightforward:
All in all, this took about ~30 seconds of work for this image.
An additional benefit of using something like Photoshop is that you can create Actions.
Assuming you got a tripod and took all of your shots from the same place using the same light - they'd all have the same borders that need to be done - allowing you to do one and apply the action to the rest, leaving only some minimal clean up efforts.
I would highly advise that, if you're ramping up this style of photography and getting busy, that you look into:
- A lightbox or similar style of area to shoot (you can make your own easily enough)
- Start learning Photoshop Actions