First off, the good sites have Advanced search capabilities that let you filter by photo, video or illustration, search for a particular color, choose one person or a group, etc. Savvy buyers would (should) use the advanced search boxes. I don�t believe that they enter common words into the search fields. String your common words together with some not so common words and you have your answers, i.e. one cow is good, cow picture is not, color cow is not, cow graphic is iffy�

The word Nobody is ok but as you say, buyers don�t use it like we do.
Anything is dumb.
Pic, picture is not needed, they are all pictures. Again, you can filter by illustration, photo, video and or audio on the big sites.
Image means nothing. Everything is an image. Everything is a picture.
Graphic is ok. Some may search for illustrations this way if they don�t filter by image type. Illustrations are sometimes called graphics by some advertising creatives.
One or single or only is good. Think like a buyer who is looking for one cow and instead gets herds of cows if he/she just searches under cow. If the keyword world was monitored, cow would/should get you one cow, cows should get you many but most put both in any image of a cow (or cows). So the word one is good.
Color is useless since many sites let you search by color. If someone wants black and white they would enter that, but not color. Colorful might be ok if the image screams many colors.