Yep its not just BMP or JPG but depends on what the native format of the renderer is. If a renderer uses 8 bit per channel (or 32 bit per pixel RGBA like many renderers do including Poser) then any and all texture files are converted to this when loading into RAM. It wont matter if your image is 8 bit or 24 bit or 32 bit they will all occupy the same space, depending only on their uncompressed resolution size. Some renderers like those of Lightwave and mentalray even use 32 bit per channel float images as native and in those renderers every pic actually gets upsampled to 128 bit images but they also have special methods to not load the whole pic at once and they can get by with much less RAM.