Maybe possible. There is file called
core\ldmain\ldlogon\policyweb\policy.portal.template.htm. It has the location of the gif:
<div id="wrapper"> <div id="header"> <div id="landeskBranding"> <img src="policyweb/landesk.gif" width="131" height="80" /> </div> <div id="portalCaption"> <span class="header2">%PORTAL_TITLE%</span> </div> </div>
However, if you point this to a network resource, what happens when users can't access that resource (it is down, laptops out of bands, etc...) Since I'm not really sure what this template does, it would be better to create a custom autofix vulnerability for landesk.gif to update the logo with your custom logo.
It turns out rebranding is not easily done in 8.8. You should be able to replace the LANDesk Logo in the LDLOGON share before building your Agent Config, but that's it.
I opened an ER with Support to have more customization available, like branding or changed text. If you want to add to this to ER, please do, and contact support:
'CR 22918 -- ER: have the ability to customize the LANDesk Software Deployment Portal adding a company logo and changing the text'