EDIT: To give you the short version, the system is saying it can't access the file you're trying to import. Below I have a few troubleshooting steps you could try.

Open the image you're trying to import in your photo viewer (Windows Photoviewer is what I use) to make sure the image isn't corrupt or locked, etc. Afterwards, close the photo viewer. Make a copy of the image and do the same thing. Reboot the system. Once rebooted, run your program and check to see if you can import the copy you made of the image. Check with other images or file types as well. The ImportImages function will work with a small assortment of picture filetypes. If the other files don't give you this issue, the file is likely locked by some other process and the reboot should have solved your issue, otherwise the file is corrupt. If all of the pictures you try to import end up giving this error, likely there is a bug in your code causing it to terminate unexpectedly.

Another thing you could do to troubleshoot whether the file is at fault, or your program, is to do a regular image import with the Laserfiche Client. If the image imports when using the client, but not when using your program, your program is likely at fault. Again, it would be prudent to reboot immediately before trying this, so as to clear any locks the file might have on it from a runaway process.