*.tga files are ordinary image files, like png, gif or jpg. If you can display it with some image viewer, for example with irfanview (
https://www.irfanview.net/) OBS will able to display it with its image source. If not, it its probably broken or in some uncommon encoding format that OBS doesn't understand. tga is notorious for its many different encoding methods, some proprietary. If you can display it with irfanview but not with OBS, use irfanview to save as jpg or png.
Since tga is able to contain alpha/transparency, it's probably best to convert to png, not to jpg or gif.