Comment 1 for bug 1325103

I have the exact same issue.

I have the panel reduced to the smallest it will go in "Panel Settings" -> "Size Options" (Allow cinnamon to scale icons... is also checked).

I've noticed that once I open pidgin, I get the blank space where the icon should be, then if I change the size of the panel, the icon appears when the panel resizes, I can then set it back to the smallest setting and it will remain. If I quit pidgin, and restart it, the issue reappears until I do something to "refresh" the icons.