use any image editing program you are good at for creating textures.
I use adobe photoshop only, many people use GIMP and even flash too.
For developing 3D meshes... IMVU supports 3DS Max, Blender and Sketchup... I heard that people use Autodesk Maya too but there is no official imvu plugin for it and people use some kind of trick or h4c|< to export from it.