I sell my work very regularly and I sign each image with a small initial digitally in the corner of the image, usually in white.
I do this per image as I finish editing the image, any prints done of the image will contain that exact signature.
I have sold USD$30k of work in the past 6 months and have not had ANY complaints about the signature being obtrusive, tacky or anything else.
I in fact get complaints if I forget to include the signature on a print, as I have done with some of my earlier work.
I'm selling to homeowners and tourists, not art afficionado's so your mileage may vary, but that's what works for me.
I have attached an image of my normal, small 16" wide size image with the signature, and a $900 Aluminium print which I sold today with the signature.
Dan