I have large grayscale texture 8bit UV mapped (23903 x 17776 pix)
It takes 2 GB in Gimp and 400 Mb of memory in IrfanView.
But when i load it in Blender the GUI frozes for 10 minutes every time when i click with mouse or what ever.
Also the UV editor frozes when i just load it as image (not UV mapped or anything).
Same happens with indexed colors image (3 unique color tones).
I have 8 GB of memory and 4 Gb on GPU, 64bit system.