Hi there,
I now have an answer which I would like to pass on, I hope it helps someone out there.
There is a plugin called WP125, it is really for carrying 125 x 125px size ads in the sidebar of your site, but with a little playing about you can get it to show logos.
For this to work well, you need to following two plugins:
1. WP125 - By Matt Harzewski
2. PC Custom CSS - By Peter Coughlin
Add the plugins to your WordPress installation.
Make your logos to a standard size - for instance 100 x 70px and a save them as a transparent png file. If you don't have Photoshop to do this, try The Gimp or an online site called pixlr, both very good and FREE.
Go back into your WordPress installation and go to PC Custom CSS, you will find it under "Appearance" as "Custom CSS"
Paste the following CSS into the page:
#wp125adwrap_2c .wp125ad {padding: 10px 5px;}
You may need to change the padding to suit your logos and you could add margin-left: xxxpx; to centre the logos on the page.
Go to WP125 plugin, you will find this under Ads, and follow the on screen details.
You can add the name of the logo, a link to the website, and in which order the ad is displayed.
If anyone has any thoughts, please let me know.
John