Importing more than 50 at a time?

Does image import only do 50 at a time?

You're going to have to code something yourself.
What kind of system would be better for that many images?

Off the top of my head, I would say either:
- something that just names each node the same as the filename
- something that looks at a simple text file to tell it the node name to use for each file, in the same sort of way as http://drupal.org/project/bones

Image import can manage hundreds at a time.
The constraint is in the machine's allocated memory ...

... or roll a one-off php script that puts things into their right spots, within drupal.

For "tens of thousands" of images, you need custom code, and ideally a developer who knows what (s)he's doing.

However, see also #993042: change number of imported images