From looking at your site, I assume you are running MG2 gallery software.
The error message is telling you that the script cannot return the filesize, because it can't find the file called for in line 400 of "home/.saphi/racesunite/2mpress.nl/gallery/includes/mg2admin_functions.php on line 400". From the error message, and the fact that I can't duplicate the error on your site, I assume that the error is created from within the admin interface (uploading a file?). A quick look at that code from the latest MG2 archive indicates makes me think the error occurred during an upload attempt.
Depending upon how that code is written, the error message could be misleading - there may be some permission issues involved if you ran the script at all while under the other user.
What did you expect them to do? It is not DreamHost support's responsibility to help you debug third-party scripts.
My advice is to log into your account via the shell, and inspect the directory and file ownership and permissions of the parts of your site that are involved with the MG2 installation, and make sure that all the files/directories are presently owned by the proper (current) user. If they are not, you are likely to have some work to do to correct that.
Alternately, given the way it appears MG2 works (from my brief look at the code), I'd consider just archiving my pictures, and re-installing it as the correct user.
[color=#CC0000]Edit and Warning[/color]:
Actually, I take back that advice! The last release of MG2 (version 0.51) was in 2005, and further development has been suspended. That seems to have followed the discovery and publication of security vulnerabilities that, as far as I can find, have not been addressed by the author or the user community.
The MG2 support forum is no longer online, and the project seems to have died in this "broken" state. My best advice is that you do not run this application unless/until you address the security issue(s).