>> Are you merging 2.6b1-mb.116 into your image or the opposite ?
>
> - I load a version of seaside in my image
> - Develop some methods
> - Wait a week or two
> - Want to publish my methods
> - Merge last seaside version in my image
> - Click on 'Changes' gives me a lot of "source same but rev changes"
>
> Hope this is clearer, sorry.

Have you tried the opposite, i.e.
1. From your image, save the Seaside package into the cache say  
Seaside2.6b1-dc.nn+1
2. Load the latest Seaside2.6-mb.116 into your image.
3. Merge Seaside2.6b1-dc.nn+1

Best luck,
Michel.