I am using the default theme. My image thumbnails are not bieng cropped, as they are all rectangular, which is the thumbnail shape we want to see. All full size images are 600 x 400 px.

My thumbnails display normally, EXCEPT on the first page of thumbnails, there is a gap of 2 thumbnails' worth of space at the left side of the second row. This kicks the last 2 thumbnails into the 4th (and last) row.

All other pages of thumbnails appear normally. Where do I look to address this issue?

Have a link maybe? And what are the values for images per pages etc.?

The site is not live. But I resolved the issue by switching from the default theme to zenpage. Now all of the thumbnails line up properly. I willl look into the problem furhter when I have a chance. Thank you for your help!

Most certainly the css not matching your thumb sizes.