Questions after a Joomla migration

Hello,

I’m migrating our website from Joomla 1.0.12 to WordPress 3.2, I know that Joomla version is too old and it sucks but the last webmaster didn’t care to even update it of course he’s not longer with us, anyway, the migration went smoothly, all the posts are there, images are viewable inside the posts, but can’t seem to find a way to convert the images to a thumbnails without editing every single post, which are more than 5000!
This is happening no matter what theme I install and it’s really frustating, so is there a way to convert all these images using maybe a batch, plugin, hack? Any pointers will be apreciated.

Thanks in advance.

BTW, this is the J! website: https://www.hardwaremx.com and this is the WordPress site(still on development): https://www.hardwaremx.com/wp/