What is the most effective change you made that consistently improved your stock asset sales?

None of the above. 

Success at Stock is 2/3 talent, 1/3 luck— being in the right place at the right time. 

Put yourself into the buyer’s shoes.

• Would you buy this?
• What would you use it for commercially?
• Is this evergreen or trending? Could I use it again in 3-12 months or never again?
• Does it contain motion? People interacting with people, places or things? Does it visually tell a story or convey emotion?
• How could it be improved?

There are no “single factors” at play, other than selecting keywords and titles and best describe the content in question. I’m not sure what you mean by improving “thumbnail” quality, as thumbnails are created automatically during the submission process and the contributor has no control over their selection. The thumbnail is the image itself when submitting images, or the first frame of a video.