When Show Link Icons is enabled in Confluence, the icon shown on links to user profiles differs based on whether the user has a personal space or not. If they do not have a personal space, the image shown is /images/icons/user_12.gif. If they do have a personal space, the image shown is /images/icons/personal_space_12.jpg. The icon personal_space_12.jpg is not a transparent image.

To reproduce this I identified one user which has a personal space and one which does not. For this example, the former is "justin" and the latter is "testuser". I then made a child page in our site's Demonstration Space with the following markup:

{panel:title=testing some transparency|borderStyle=dashed|borderColor=blue|titleBGColor=#00a400|titleColor=white|bgColor=#72bc72}
[~justin]
[~testuser]
text
{panel}

The results are:

I recommend that this image be replaced with a transparent equivalent.