Also consider...

These days, most Linux distributions (distros) ship with a basic image viewer that can be used to read and edit metadata and tags, and will likely offer some basic image-editing options. If not, then take a look at your distro’s standard repository (repos), which will be full of helpful tools that can help you sort through all your Stallman snaps.

Even basic image viewers, such as Geeqie and gThumb have organisational and editing capabilities. If you shoot a lot of RAW images you should perhaps look at RawTherapee and GTKRawGallery that are designed as per the workflow of a professional RAW developer. Finally, if you’re used to Google’s Picasa, which used to have a native Linux version, you can use the Windows freeware on top of Linux via the wonderful Wine.