123RF is the agency where the two numbers collide hardest. Its upload guide requires photos to have a resolution of no less than 6 megapixels and a file size that does not exceed 30MB, in JPEG format and at minimum 80% quality compression. Its contributor FAQ is blunter about the first rule: images should be at least 6 million pixels and anything lesser will be automatically rejected. Six megapixels is roughly 2000 x 3000, which most modern cameras clear comfortably — the files that fail are crops, phone exports and downsized web copies submitted by mistake. The 80% quality floor is the part that changes how you compress. A 30 MB ceiling on a 6 MP+ file is a soft ceiling in practice, because a 6 MP JPEG at 80% quality lands far below 30 MB. That means quality is the binding constraint for most submissions and bytes are the constraint only for very large files. Compress in small steps from 100% down, stop at 80% and check the byte count there; only go further if you are still over 30 MB, which on a 6-24 MP file you will not be.