All personal computers (Windows, Mac, Linux) already have built-in tools for viewing most standard picture formats: JPG, GIF, BMP, PNG.
A web browser alone is sufficient, but specialized tools are also available with your OS. Indeed, a lot of email clients use web browser technology for the purpose of rendering your email message, complete with inline pictures.
If someone is emailing images to you and you cannot open them, then perhaps they’re using a proprietary format like Photoshop PSD format?
Can you provide more details?