The fact is: there are no a lot of graphics software which can load really large images and view it in its original size without problems. The Zoomer is very rare software which can do this work for you.
For images of jpeg, png, gif and tiff formats (if this last is not encoded with a division it onto tiles or strips), the image is first re-packed into a temporary file on disk. This takes some time and require a lot of disk space, but this should be acceptable, since there are no other way to handle images with size bigger than 128 Mega-pixels. During the repacking, the progress bar is showing to allow you stopping the process if you have no time to wait. But when the loading is finished, you can see the entire image fast enough, scaling it, rotating it, scrolling, gamming, cropping etc.
For not compressed bitmaps and tiffs divided onto tiles / strips, things are even easier: the image is shown directly from the place where it is located. Since such kind of images can be unpacked by parts, and only those parts of the image what are necessary to show on the screen are decoded.
To get know how many disk space is required, multiply width by height of the image and in most cases this will be a size of temporary re-packed image (actually, it is necessary to multiply by 3, since each pixel is represented by 3 bytes to store R, G, B channels, but the image is stored on disk packed with fast lossless scheme, which makes it smaller about 3-4 times versus the size of the unpacked image). E.g. the image 30,000x30,000 will occupy approximately 900 Megabytes (or less if you were lucky).