No instructions in Custom Icon tab of Icon Browser.

[AdonisNimbus]

As the title says. The site here says to follow the instructions in the custom tab of the icon browser to add new icons. The only things in the custom tab are the refresh button and the open icon folder button. I have no idea what format of image to place into the folder, nor their limitations, because the supposed instructions are simply absent. Just looking to add some custom venue icons for shouts relating to RP venues, but I don't know what the means are. I tried PNG and GIF images to no avail, and thought I'd come check here for some assistance.