What would be the optimum size of a background image? I get them often so blurry, that it is difficult to reproduce a linesplan, and sometimes so big, and accurate, that it slows my computer down. ( Sempron 3200, 1 GB, 160 GB)
Optimum size would be the same size as your screen. However I mostly use about twice my screen width, so the image still has qood quality when you are zooming in on it. So in case of a resolution of 1024×768 pixels, make your image 1750 to 2000 pixels in width.
Just remember, the size of your image on disc has nothing to do with the actual size when loaded in memory. Each individual pixel uses 3 bytes in memory. So an image of 2000×1500 pixels uses 2000x1500x3=8.6 Mb when loaded, while it may use 300kb when saved to disc as a jpeg image for instance due to the compression.