Dear Josephine,

you are making one big mistake (and one or two smaller ones):

The "adding" of new bricks -as you imagine it- is NOT possible. If you want to add a new brick you always have to REPLACE an old brick with it, using the old brick's res ID.
This is because Vern did not provide the "empty space" needed for additional bricks / tiles (and additional res IDs)...which you can see very easily in MM's Level Editor: The top row displaying all available tiles and sprites is already full! Where should a new, additional res ID appear ?  
So you have to replace an existing res ID's image with your new PICT, which does not mean however, that you're restricted to simply changing a brick's color.
If you look at Joe B's set of graphics or my Discotheque of Doom custom bits you'll notice that we have changed some of the tiles + sprites quite drastically...within certain limits.

Example: you cannot change a normal "brick" into a "moving platform" sprite or something. You also cannot change a blue bird into  - say - a tiger....because that tiger would then fly; which is due to the fact that Vern has programmed a certain amount of "behavioural traits" (my expression) for the sprite "blue bird". You can replace its image with that of a tiger, but it would still behave like the blue bird.

What you can do, however, is change blue or gold bricks into sand or snow tiles, for example, or those "oil paintings" into pop art posters (as I did in DoD), or even change Jack into a robot as Joe so successfully once did.
That last thing, trust me, is a huge amount of work though.

So start with simple things.

The second, smaller mistake you apparently make is that you put your new custom PICTs into the "Midnight Mansion Sprites" res file, which is no longer needed. You can put it directly into your mansion's res file. But take care to apply it the exact res ID as the tile or sprite you want (it) to replace (which of course DOES reside in the MM Sprites file).
However, you don't have to remove the old res ID, as the game is now clever enough to know your having added a PICT res with the number XXX into your (mansion name) file means this is the one you want to have used (instead of the old one).

Clear enough ?