Frame extraction is fast — typically 20-30% of source duration on the standard pipeline. A 5-minute MKV → JPG bundle finishes in about 1 minute regardless of frame count, because the bottleneck is the JPG encoder, not the demuxer.
Yes, subject to whatever licence governs the source MKV content. The format change adds no claim — we add no watermark and claim no licence on the JPG output.