Card Cropper (Front & Back)
Upload your screenshot once, then drag/resize the two colored boxes Front and Back right over the cards. The tool crops both and gives you downloadable PNGs. Regions are saved as percentages so it works with any image size.
Front (blue)
Back (green)
Click a box to activate; drag to move, use handles to resize.
Defaults tuned for an example 540×1200 layout; adjust visually if your layout differs. Tip: hold Shift while dragging to move by larger steps.