If you're looking to export a single frame from Final Cut Pro X as a JPG, there's a built-in function to that. It takes the current frame at the playhead, and it works with layers and titles and any other visual effects you've applied.
1: Position the playhead at the frame you want to capture
2. From the top menu, choose File > Share > JPG Current Frame
3. Fill out the title, description, creator, and tags fields
The title is what's used for the filename. The other fields are entirely option--leave them blank if you like.
4. Hit the Settings tab and choose the file format for the export.
By default, it exports as a JPG.
There are also some other options aside from JPG. You can also choose TIFF, PNG, PSD, OpenEXR, or DPX image file formats.
There's no way to specify dimensions when you export. It just takes the resolution of the project and uses that.
The colorspace is taken from the embedded colorspace of the source project.