I did some more testing and think I found what caused the image importing problem:

I have a G5 1,8 (single CPU) and 1 GB of Ram.
Now, since the update to 10.3.3 , I found my images imported to be corrupted/damaged. On my girlfriends iMac and also 10.3.3 everything was ok. It doesn't matter if I used iPhoto or other ways to import the pictures. The images and CF Cards are ok, so I concluded it must be something specific with my G5. I even tried 3 different digital camera's (Canon G5, 300D and a Nikon D70), but all with same results.

So next step was testing the system. Since I have 2 hard drives and several partitions, I tested 10.2.8, 10.3 and 10.3.3.
The problem only occured when using 10.3.3. So I assumed it was a 10.3.3 specific problem.

I searched at Google for people reporting the same problems, and found a few similar reports on http://www.xlr8yourmac.com/OSX/10.3.3_feedback.html#cameraimports and http://www.xlr8yourmac.com/G5/G5_usb_firewire_reports.html#imaging
Some users reported relevance with memory modules, so I did some more testing. Using the basic Apple Hardware test and Techtool Pro, memory seemed to be ok.

But the extended Apple Hardware Test reported a memory error, so I thought my third party memory (good brand, lifetime warranty) must be corrupted. To be sure I tested those memory moduls without the original Apple memory. (using extended Apple Hardware Test) But no problems found...! Testing the Apple memory without the other memory, also no problems found...!

Both memory moduls seem to be 100% ok, but not when using them together! They have exactly the same specifications, so I guess minor deviations can may cause problems on the G5!

Also, when using only 512 MB and not combining the two different brands, importing the images ran without problems even in 10.3.3...! Brand doesn't matter.
My guess: it somehow seems that 10.3.3 uses a changed I/O protokol, using some other memory specific algorithm. Therefor the "image importing" problem was not noticable on earlier systems, and only shows if there is a "problem" with the memory.

My conclusion: actually there is not really a problem with either 10.3.3, USB, importing images, or CF cards.
10.3.3 only seems to make some memory problems noticable.

The actuall problem seems to be: the G5 has problems dealing with mixing memory of different brands or minor variations...
I believe the importing problems are only pointing to some kind of memory problems (defect memory, or mixed memory in my case)

If I'm right, then I really hope this is a known problem at Apple and it can be fixed by a firmware update (soon).
(If not, I probably need to buy more memory again, exactly same type and brand)