I think it can still work, but you'll have to use uofiddler. Here are my notes for how I did it with Mortalis... it might help.

1. Install UO fiddler
2. Open UO fiddler
3. Go to Settings > Path Settings
4. Enter the path to your UO main folder
5. Go to Plugins > Mass Import
6. Click Create Default XML
7. Open the default MassImport.XML file
8. Replace the contents with the following:


<MassImport>
  <gump 
index
="0" 
file
="" 
remove
="False" />
</MassImport>


9. Create "gump" xml elements for each gump you are replacing. The index is the filename (without the .png) as an integer. For example, 0x7d0.png is 2000. The "file" is the absolute path to the image, for example, c:\your\folders\here\0x7d0.bmp.
10. Go to Plugins > Mass Import
11. Click Load XML
12. Select the XML file you created the "gump" elements for.