When doing html to image conversion with html snapshot, the image will be rendered with current display context.
So to achieve the best image quality, the display mode needs to be in true color mode, either 24 or 32 bits. Almost every video card can support the above mode nowadays.
The other thing related is the remote desktop issue. The remote desktop supports desktop display in 256 color mode, this has the effect of setting the remote display to 256 color mode. Be aware of this when doing html to image on remote server with remote desktop.