First off, we should clarify what an image mosaic means, here. A mosaic consists of several images combined to a larger image. When combining images to a mosaic using XnView, the images will be aligned horizontally or vertically and do not overlap.
If you want to combine your images randomly and overlapping, you cannot do this with XnView. You need a real mosaic software to achieve this. Please search the web for the terms "mosaic graphic" and you will find various software.
Back to XnView:
If you want to create one large image out of several, different images, you can use "Create->Strip of images...". XnView can merge several images merge together to one. XnView aligns the images side by side and merges them to a larger image. You can align the images horizontally or vertically.
In the XnView online help, chapter "Create panorama images" you will find interesting info, here an advanced sample is shown:
E.g. to create a 2x2 image mosaic you have to apply "Create > Strip of images.." three times:
- First merge (horizontal):
- Second merge (horizontal):
- Third merge (vertical)
Make sure that the checkbox "Automatically adjust height or width, respectively" is checked. Then, XnView will rescale the images when aligning. This will ensure that there is no border if images differ in size.
Four a 9x9 image you have to do 10 merges (9 horizontal, 1 vertical). It's many steps but not too complicated. But if you want to create many mosaics, I think you better look for a software which provides a special function to do this.
Most of this info is taken from post 'Combine images to mosaic'.