Contributor submissions are batches, not single images. A night's culling produces 20, 50, 200 files that all have to clear the same agency floor and the same byte ceiling, and doing that one file at a time is where submissions stall. The published numbers are per file, but the practical problem is per set: Dreamstime wants JPG in RGB or sRGB between 3 MP and 70 MP, Depositphotos wants at least 3.4 MP under 50 MB, 123RF wants at least 6 megapixels under 30 MB at 80% quality or better. Compress against the strictest agency in your distribution list and the same export clears all of them — 6 MP, under 30 MB, JPEG, quality left high. Two habits make a batch survive review. First, compress from the full-size master and never from an already-compressed copy: recompressing a JPEG re-encodes every block and the artifacts compound, which is precisely what the agencies reject. Second, check the megapixel count of the whole set before you start, because compression will not rescue a crop that fell under the floor. Batch compression fixes weight. It cannot fix pixels.