Why are web images usually unsuitable for raster to vector conversion?

The logo below looks absolutely fine. It is clear and legible.

Logo

You may think you can just download it and vectorize it, but if you try to vectorize this logo, you'll find it won't work. Why?

The main reason is that it is too low resolution.

If you read the logo into your raster to vector converter, into Photoshop or into any program that reads raster images, then zoom into it, you will see that although the image makes sense when viewed on the web, it doesn't make sense when viewed close up.

For example, here is a zoomed in view of the "r"; from the logo above.
 

Too low resolution for raster to vector conversion     Desired vectors following raster to vector conversion
The letter "r" from the logo above, zoomed in     Desired vectors

If you did not know this was an "r", you could not say, this is an "r". It doesn't look like an "r", it looks like a blob. There are not enough pixels in the "r" to adequately define its shape - for example, there are no curves. A raster to vector converter would be unable to create the desired vectors from it.

Web images are of necessity very low resolution.

Web sites are designed to be viewable on most peoples' screens.

Like raster images, screen displays are made up of squares, or pixels.

Our web use statistics tell us that the majority of people visiting this web site are using screens with 1024 pixels displayed across the screen area and 768 down it. Most web site designs, then, are made up to a 1024 pixel width size or thereabouts.

As a logo on a web site normally takes up only a small portion of the web site's design, it will be considerably smaller that 1024 x 768 pixels in size. The logo at the top of this page for example is 120 x 80 pixels.

The "r" is only a small portion of the 120 x 80 pixel logo - about 9 x 5 pixels. To get a good conversion of the "r", you would probably be looking at it being made up out of about 150 x 200 pixels:
 

High resolution raster image suitable for raster to vector conversion
High resolution raster image with well-defined outline and curves

The bottom line is that images on the web are designed to look good on the web. An image that looks good on the web needs different attributes from an image that will convert well to vectors. Most specifically, an image on the web will almost certainly be too low resolution to convert.