As long as the images are formatted as a .png, .jpg, or .webp and placed in the "portraits" or "bodies" appdata folders, it will work. If you want a better fit or match, more details can be found at https://itch.io/t/838806/core-tag-size-rules. If you want a transparent background, then you will either need to do it yourself or find someone on the Strive Discord (https://itch.io/t/284398/discord) that can do it for you. Seems that most people edit images using either Gimp or Photoshop and the fuzzy select tool. More detailed explanations can be easily found by searching the Discord for "fuzzy".