Upload the 3GP file and the converter exposes a frame picker: every Nth frame, frames at explicit timestamps, or one frame per second. Each chosen frame is encoded as a separate JPEG file and bundled as a ZIP for download.
Frame extraction is fast — typically 20-30% of source duration on the standard pipeline. A 5-minute 3GP -> JPEG bundle finishes in about 1 minute regardless of frame count, because the bottleneck is the JPEG encoder not the demuxer.