think about it man, if vipers do make a mental image of prey, its only a blob of heat, think about how heat travels, and its impossible to imagine heat being able to make a picture, even if the heat pits were connected directly to the brain.
The "blob" effect you see on TV shows is relative to the resolution of the device used. The better the device, the better the resolution and the resulting image. Stating that the snake only sees a blob is silly without proof.
The following image was taken with infrared photography:
Assuming that a creature that detects infrared has adapted to visualizing in that mode, the limits at which they can view the world are only defined by the hardware limitations of their own organs.
edit: had to change photo links, as other photo wouldn't load.