An idea creating DOOM monsters

[Vegeta]

I'm looking for a paint program that allow to create inter graphics from two existen ones.

I'm not a wizard with English, and I don't the name of this operation.

Let me explain.
In Example, you have a picture A, and a picture B, using this utility you can create a picture that is in the middle of both (like in B&W from Michael Jackson).

I put this here, because I'm planning to use it for create a new DOOM Spider monster, crossing the Spidermastermaind and the Arachnotrom, also I think that other doomers can use this idea for new monsters, textures, or whatever, and then insert them in their wads.

If you understood what I mean, can you help me, suggestin an utility, or at least telling me the name of this operation? Please.

[exl]

I think that what you're talking about is called 'morphing'. There's a few (freeware) tools on the web, but I don't know their names.