Which blocks can't be covered in snow?

There is no "heat" in minecraft. Lava melts snow because it is a light source. In the case of stone above lava, no light will pass through, so it will not work.

From the wiki:

In snowy biomes or in cold biomes at varying layers, it will snow instead of rain, and while it is snowing, snow will generate on random non-transparent blocks with a block light level of 10 or less, with the exception of packed ice.

The wiki also contains:

Snow will melt if there is a block light level of 12 or more

This had me a little confused as I thought I had seen snow generate on glass before. I built a creative test world using 1410403532 for a seed to get a snow biome.

I placed down several blocks including light blocks, partial blocks, and glass. I then made it snow with command: /weather rain 600. This causes snow for 10 minutes.

The first thing I played with was light levels. Snow will not fall onto a block with a light level above 10 as stated.

If you then cover the light with snow or place a light source in an already snow covered area, it will only melt back to a light level of 12 which is a smaller patch then the area that won't be snowed on if already cleared.

I tested glass to see if I remembered correctly. It did snow on the glass. (Note: I used a texture pack to make the glass clear)

I tested slabs both upper and lower. It will snow on upper slabs as the top surface of the block is solid. It will not snow on lower slabs.

I tested several blocks that don't have a solid top as well as ice. Blocks without a solid top will not be snowed on. Packed and solid ice will not be snowed on. This is stairs, fence, tracks, pressure plates, grass, carpet, trap doors, and both types of ice.

You can also build a platform above the mural. You can build it at very high levels so it is likely to not be noticed up in the sky while looking at mural. This will prevent snow from forming on any block below it so you could build the mural out of any desired block.