Where do I get usable Icons for VB?

I'm building my "learning" application in VB, and it requires the use of icons on toolbars and panels. I had a few icons already, and downloaded a whole bunch more from a free website.

My problem is that when I try to set some of the icon files as the image in a picture object I get an error telling me that the file I have selected is not valid. Others, with identical file extensions and apparently similar characteristics work fine.

I would not even mind paying to buy an icon library, but would like to be sure that I can use what I am buying.

Thanks!