I just wanted to drop this here for anyone else that may run into similar issues. In a nutshell, uploading logos in portal manager would only sometimes sync to our clients...it typically resulted in a blank logo. I know that the recommended file type is .png, but I haven't found any relevant information regarding file size (which doesn't matter, as I've discovered).
Resolution: If you pre-populate the logo on the core in x:\Ivanti\LdMain\landesk\files\SWDPolicyImages, *then* upload the logo image via distribution package (use the same name, file type) - you can safely trick the core into properly accepting the image, then syncing to the clients. This issue was not seen in EPM 2017, it was just noticed in EPM 2018.3.
Anyways, hope this helps someone out there!