Is there a tool to migrate Joomla/K2 entries into Craft? I have a Joomla site with K2 installed, and a couple of thousands of articles (no custom fields) and corresponding images.

Is there a script to help migrate those into Craft? Or will I have to manually write something custom to move the content?

Thanks in advance
I'm not sure there is anything specifically for importing from Joomla, but you could try the Import plugin. Depending on the data format, you may also be able to use the Feed Me plugin.

Edit: Alternatively, a popular Craft CMS plugin maker (Barrel Strength Design) offers a commercial service to migrate/import data from Joomla -> Craft CMS. It's called The Great Migration. (NOTE: I'm nothing to do with the service, I just had it bookmarked for future reference)