Let's see...Selecto Bannershop GIF animator 5 is not the best one around, but it does the trick. It's not free, but you can download a 30 day trial version
here. The "Server 1" link on the site seems to work better.
Just PNG? Why does it matter? Bannershop GIF animator animates image files with (non animated).GIF, .BMP, .JPG, .TIFF, .JP2, .IFF, .PCX, .PSD, .RAS, .RSB, .SGI, .TGA and .WBMP extensions along side .PNG, so you should have no problems.
Alternatively, you could use one of the more professional programs to animate. Adobe Imageready works excellently and is great to work with, but the price is amazingly high, so if you plan on using it for only one animation, don't buy it.