Ah yeah, despite it being a 1x1 transparent png, there are some CSS margins in place that are causing the additional space.
It sounds like this could turn in to a feature, but in the mean time, this should be the CSS to completely hide the images (and their margins) for your categories that don’t have logos:
// Hide category logos
[data-category-id="24"], // Weekly Challenge
[data-category-id="20"], // Technical
[data-category-id="59"], // Other Discussions
[data-category-id="40"], // Field Reports
[data-category-id="31"], // Classifieds
[data-category-id="33"] { // Community Events
img.category-logo {
display: none;
}
}