This is my self experience and I am posting here as it goes. I wanted to open a .tga file extension format in my Windows 10 laptop. This was the file which was found in the Warcraft 3 directory, when I took a screenshot of the game by pressing Print Screen button while playing. It stored inside the game folder > screenshot folder.
Update: You can download Quicktime player from here: http://filehippo.com/download_quicktime_player/, and it will open the .tga file. All you need to do is double click on .tga file and select PictureViewer with Quicktime icon. It has only less than 40MB file size for download.
I have tried to open it with MS Paint but no use. Later I have downloaded the GIMP open source image editor from the official website. It's around 75MB and it opened my screenshot, which can be later saved or converted into jpeg or png file as well. This information could also answer the questions like how to convert TGA to PNG, How to convert TGA to JPEG etc. Download GIMP from here.