Displaying thumbnail icons 128×128 pixels or larger in a grid in ListView

For update:

1. Set image list color depth in addition to image size (ilist.ColorDepth = ColorDepth.Depth24Bit)
2. WinForms ListView does not have possibility to change icon spacing, however it can be easily done using Win32. You need to send LVM_SETICONSPACING to your ListView (there is a lot of tutorials how to use SendMessage win32 function in .net, so I think this direction must be enough for you).