How to Create VERY BIG IMAGE ? 30000*30000 DPI,24

Consider the following

The most common file formats store their data row wise, as scan lines. Either top down, i.e. row 1 followed by row 2 etc, or bottom up, last row first, followed by second last row etc. These methods are efficient if you want to display the entire image at once.

If you create a very large image, these scan lines will be very long. As a result, if you want to display a small portion of the image, data will be scattered over a huge memory area, thus making it slow to render. There are (at least) two solutions to this. Either you go the Christian Graus way, and create a number of smaller images you stitch together while rendering, or you chose a different image file format which does this internally. E.g. the RIK file format does this. There are probably others as well. There is always the option of creating your own file format if you only want to display the file in your own application.

On the other hand, if you really want a huge jpg encoded image, you will probably have to encode it on the fly writing data to disk. The resulting image will probably be close to unusable due to its size.