Thumbnail sizes help. Different thumbnail sizes in media browser

Hi,

My thumbnails in wordpress are quite messed up and I am very confused. I basically want to display 172×88 thumbs on my homepage and on my category pages. On my homepage they are constrained by the CSS at the moment (mainly because I cannot get the sizing to work).

On the category pages they are mainly okay, but sometimes they are weird sizes like 61×88. The 88 seems to work but the 172 goes wrong all the time…

I am totally lost. I recently added:

if ( function_exists( 'add_image_size' ) ) {
	        add_image_size( 'cat-post-thumb', 172, 88, true );
	}

To my functions file when I coded the homepage, I also changed the media thumbnail size to 172×88 in the admin. I have changed settings everywhere, messed about with everything I could find and read the documentation but I do not understand where these other sizes are coming from.

I know these things:

When I upload media images are saved at:

• filename-172×88.jpg
• filename-88×172.jpg (a dup??)
• filename-100×88.jpg
• filename-300×262.jpg
• filename-556×198.jpg
• filename-originalsize.jpg

I am not always shown the 172X88 size in the media browser (sometimes I am) but also sometimes 88X88?

I really want to understand what is going on here, and fix this as it’s driving me mad!

Thank you in advance.