If you ever need a LOST screencap, xlostcapsx.com is the place to go.
“Screen captures on this website are taken from Blu-ray at 1080p HD, 1920x1080 pixel dimensions. The promotional and episodic stills archived here are no less than 3000 pixels on the largest dimension.” Quality? Check.
There are at about 1,000 caps from every episode in easily downloadable zip files. Quantity? Check.