I was reading on some forums that it's possible to include thumbnails in the info-tip.
[quote]Settings/filetype/all files/edit/info-tip
should include "{thumbnail}" (no inverted commas!)
You may have to activate plugin in preferences as well but i dont think so[/quote]
However I did this, but nothing shows up. I can't find any plugin that would require this. What exactly am I doing wrong, or is it no longer supported?
I put my {thumbnail} code under the Images group, not All Files, and it's always worked for me.
Yeah I just realized it's only for list/details mode. I was hoping it could work in tiles mode to give a larger thumbnail preview than the default size, but {thumbnail} isn't support in tile mode.
Is there no other way to create a mouse over effect that displays a custom sized thumbnail?
Someone brought this up back in 2007. I just recently put my vote in for just such a feature. Put your vote in, maybe we can get it moved up on the to-do list 
Just post here on the forums, that you like the idea, to get your vote counted.
It works in tiles mode as well (as of Opus 10.1.0.0), but only if the thumbnail is significantly larger than what's already shown in the tile (otherwise it'd look silly showing almost the same image in a tooltip over itself).