There seems to be a pretty recognizable and consistent artistic image of Jesus Christ, despite the lack of evidence of what he looked like. Where did this initial image originate?
Popular Mechanics (of all sources) did a good article on "The Real Face of Jesus".
The article discusses how the current representations of Jesus came to be. The European image with long wavy brown hair that we're all familiar with in Western cultures isn't universal at all. Rather, it seems to simply have been an image that artists rendered, and that we, as a culture, find pleasing. He looks Kind, loving, gentle, and everything we know Jesus was, but it it, of course, just an artist's interpretation, copied and emulated over the years.
From the article: (emphasis mine)
Hopefully, the above will do for an answer. I know it's vague. For a more specific answer, I believe you'd have to find the earliest representation of Jesus that you feel represents the western image we're used to. This would be a difficult thing to do, and still meet with the site's strong bias toward objective answers (or those based upon commonly accepted canon or a specific doctrine.)
Until at least 400 AD, Christians were reluctant to make any image of Christ, due to idolatry. Occasionally Christ was pictured as a lamb, as He is several times represented in the Bible.
An image at Edessa began to be regarded as a likeness of Christ, and was said to have been made by Christ himself. Church leaders contended that it was therefore authorized to portray the face of Christ, and that picturing Him as a lamb was inappropriate.
While artists used their own imagination of how Christ's features should be drawn, many of them appear to have used the Edessa image as a pattern or been influenced by others who had. The image on the shroud of Turin looks similar, and it is believed by some that it had been folded to show only the face, and was in fact the Edessa image.
Here is a blog post by a Roman Catholic about the Edessa image, which he firmly believes is also the shroud: