Interesting. I am also using captive portal on the controller and I am using a logo image of size 372x148 and it seems to work fine for me (I am running v3.4.10). In the past when I had some issues with the logo not showing up it was due to the way I imported it on the controller. I had inadvertedly selected the wrong "page type" so the logo did not show up.
I also tested the file size under AOS version 6.1.3.7 and the largest image size that works is 176x46. So cjoseph if right there is a change in the newer OS version with regards to the image size restriction.
These are the steps I used to import the logo image on my controller:
1. make sure you are selecting the right profile from the drop down
2. browse to locate the image file to be imported
3. set the "Page Type" to "Content"
4. Apply.
I hope this helps.