I use an Epson WF-3540 scanner and an old version of PhotoShop(CS2)to reduce 1200dpi scans to 200Kb files. PhotoShop, and likely other photo programs give options for "quality" when saving a jpg image. The following image of the $1 Franklin was made at 1200dpi, originally a 2.24Mb file, but saved as a 201Kb file which was easily accepted by the website.
This image (below) of the PhotoShop jpg save function shows the quality options (in this case 5 which produced a 357Kb file). The image shown was saved as a quality level 2.
Anyway, that is my way of producing quality scans. Hope it might be helpful.