As vizmaster already asked, is this a static texture or a dynamic texture that is being updated at some real-time rate? How much memory does your graphics card have?
If your graphics card supports texture compression, then you can greatly reduce the amount of texture memory required. However, texture compression will require your image dimensions to be a power of two (512,1024,2048,etc..)
Here is sample code to compress a texture with the highest compression available:
Code:
texture.compression(viz.COMPRESSION_DXT1)