Blanks are models presized to fit over the Valve ads. The model coverage is given in the model name.
So a model named 'blank128x256.mdl' will have a coverage of 128 x 256 pixels. To use the blanks,
open the model in a model viewer ( I use 'Jed's Half-Life Model Viewer') and import the desired texture.
Note that the texture must be the same size as the one you're replacing, and use 8-bit color mapping.
The file 'adthemes.txt' gives the sizes and location for the currently used ads (as of 3/27/07).

There are 3 styles of blanks...
Standard - These models' textures are the same size as the model itself. Good for text or simple pix.

Hi-Resolution - Designated by 'hr' following the size, these models' textures are of a higher resolution
  than the standard models. Use these if you want more detail (less pixelation) when displayed.
  All 'hr' testures are 200 or 400 pixels in width or length.

Hi-Resolution Offset - Designated by '_os##' following the 'hr'. These models are the same as their 'hr'
  counterpart except they have a 'Light_Origin' offset built into the model. This allows the origin of the
  model to be in a brighter area than the texture. The light values at the origin is then applied to the
  texture. The location origin of these models must be adjusted to account for the offset. The theme
  'adthemes_none.cfg' uses these offsets.