I agree it’s going to be near impossible from a photo. I used this ring sizer to get my fiancées.
TANJIN Black US UK Ring Sizer Measuring Tool Plastic Finger Sizer Ring Gauge Measuring Tool Belt for Measuring Wedding Rings Proposal Rings 2 PCS https://www.amazon.com/dp/B07TLB4GPV/ref=cm_sw_r_cp_api_glt_fabc_39HV3RZ6WMYTVT7CTSVG?_encoding=UTF8&psc=1
If you have printer I think Cartier has a chart you can use! Otherwise you can probably get a ring sizer like this! There’s also another chart for conversion!